Yaakov Taubes
The current year―2024 on the civil calendar and 5784 on the Jewish calendar―is a leap year for both calendars. The civil “leap day,” February 29, falls during the Jewish “leap month,” Adar Rishon―a situation which will surely cause chatter and excitement among calendar enthusiasts.[1] When I was in middle school, I remember one of my rebbeim, with a certain air of superiority, telling us that while the secular/civil/Gregorian calendar has only a leap day, our Jewish calendar has a whole leap month! I assume that this comment was made somewhat in jest, as there is nothing inherently greater about having more leap days; quite the opposite is true if we understand that the addition of a day or a month is a means of correcting a potential flaw in the calendar. Still, the basic point that the Jewish calendar adds a full month every so often, while the civil calendar adds only a day, remains true.
February 29 has become associated with all sorts of lore and special customs, and while the leap month on the Jewish calendar has been pondered and discussed by various Jewish thinkers and legal authorities, the two have apparently little in common. Unbeknownst to many, however, from one perspective there is indeed one specific day in the Jewish “leap month” that is more of a leap day than any other and thus more akin to February 29. To understand why, we need to first briefly review how the Jewish calendar works.
The Jewish calendar is primarily a lunar one, meaning that the months are determined based on the cycle of the moon going around the earth; accordingly, the Jewish holidays are all a function of those months. But it also incorporates elements of the solar year, calculated based upon the amount of time it takes for the earth to revolve around the sun, which is the determinant for the four seasons―and in this way, it has relevance for Jewish practice as well. The holiday of Passover is also known as Hag Ha-Aviv, the spring festival, and while the Torah specifies the specific lunar month in which it is to be observed (the first month, now known as Nissan), we are also taught that it must take place during the spring season, meaning after the spring equinox, which occurs nowadays toward the end of March.
The lunar calendar year generally lasts 354 days (12 lunar months of either 29 or 30 days, the cycle of the moon’s rotation around the earth being approximately 29½ days). Since the lunar calendar is 11 days shorter than the solar year, which lasts slightly more than 365 days, the holiday of Passover would fall 11 days earlier each year on the solar calendar and eventually not occur in the spring at all. To avoid this problem, the Jewish calendar requires the addition of an extra month to the years in which Passover would fall out too early. The Talmud mentions other circumstances under which it is also appropriate to add the extra month, including situations involving crop shortages and impurity of the masses which could preempt their bringing of the Korban Pesah.[2] Regardless of the reason, this type of year is called a shanah me’uberet, literally a “pregnant” year, but is colloquially known as a leap year.
According to many authorities, the calculation of exactly when to add the extra month is an independent biblical commandment that must be fulfilled by the Beit Din, or rabbinic authorities, similar to the commandment to calculate and hear witnesses concerning the appearance of the new moon and the declaration of each individual new month.[3] Once a set calendar was established (by Hillel the Younger in the fourth century, according to a tradition first recorded in the Iggeret Rav Sherira Gaon), the extra month was incorporated into the fixed cycle of years. This is how the Jewish calendar functions to this day, with leap years occurring seven times in 19 years, or approximately once every three years.
The extra month was added specifically at the end of the lunar year, before Nissan, in order to provide the rabbinic authorities requisite time to determine whether the extra month is in fact needed and so that it would be clear why the month is added in the first place; this remains the practice today. The last month prior to Nissan is Adar, and this is why we have Adar Rishon /Alef and Adar Sheni /Bet in a leap year. Already in the Talmud, the issue as to which Adar is the “real one” is addressed when discussing the question of in which month one should celebrate Purim, among other things, during such a year.
According to one view, Purim should be celebrated in the first Adar due to the concept of zerizim makdimin le-mitzvot, meaning that one should try to fulfill all the mitzvot with alacrity, and observing the holiday at the earlier time is thus preferred. The second view is that Purim should be celebrated in the second Adar since we want to juxtapose the redemption celebrated on Purim with that of Passover during the following month. The Gemara is therefore not necessarily indicating which Adar is the primary one, as neither explanation is connected to the identity of the “true” Adar. Consequently, the accepted pesak that Purim is celebrated in Adar Sheni does not necessarily mean it is viewed as the true Adar for all other areas of Halakhah. Still, it demonstrates that already in Talmudic times, the question as to during which Adar should month-dependent practices be observed was subject to dispute.
There are several ramifications to this question beyond Purim that rabbinic authorities have discussed for generations, including commemorating a yahrtzeit (the day a person has died) and a bar/bat mitzvah (the day a boy or girl becomes considered a halakhic adult). Specifically, if a person died or was born during Adar of a non-leap year, when should the yahrtzeit be commemorated or the bar/bat mitzvah be marked during a leap year?[4] Many of these issues have already been dealt with in depth elsewhere, and it is not the purpose of this essay to review those discussions.[5]
There is, however, one permutation of this question that is somewhat less well known or discussed. To return to our Jewish calendar review, the moon takes 29½ days to rotate around the earth. Since the calendar does not allow for one half of a day to belong to one month and the other half to another, that half must either yield an extra day in the old (or outgoing) month or be part of the new month. In Temple and Mishnaic times, this decision depended upon the arrival of witnesses who would testify as to having seen the appearance of the new moon on the night following the 29th day of the present month.
If the moon had indeed been visible and seen by witnesses who were interrogated by the High Court for accuracy the next day, then that day would be declared day 1 of the “new” month. If this did not happen, it would instead be day 30 of the “old” month, and the next day would be day 1 of the new month. This is why each month on the Jewish (lunar) calendar is either 29 or 30 days, never more and never less.[6] Once the calendar was set, the determination as to which months would have 29 days and which would have 30 was fixed (with a few exceptions that will be discussed below). When the outgoing month has 30 days, the last day of the month is observed as Rosh Hodesh along with the first day of the new month; when the old month has 29 days, only the first of the incoming month is observed as Rosh Hodesh.
According to our current calendar, the month of Adar in a regular year is always 29 days long. If the extra month, be it Adar Rishon or Adar Sheni, is still Adar, we would expect it to likewise be a 29-day month. In fact, however, while Adar Sheni is indeed 29 days, Adar Rishon is actually 30 days. This effectively means that the “30th of Adar” does not exist as a calendar date in most (i.e., non-leap) years, making it the true “leap day” of the Jewish calendar, just like February 29 on the solar calendar! Those born on February 29 or 30 Adar Rishon can claim with equal gusto that they have fewer birthdays than others and are therefore years younger!
On some level, of course, this claim is absurd. A person ages the same regardless of when his or her birthday is celebrated. There are, however, some legal ramifications regarding February 29, some of which vary slightly by country and even by state, at least within the United States. Generally, under American law, February 29 is considered a “day” when the particular law is statutorily specified in terms of days, but not when the relevant law relates to years.[7] This has led to a few court cases in which appeals were filed to determine if the time stated in the law referred to days or years. In most states, a person who is born on February 29 is legally considered to have their birthday on March 1 in non-leap years, although there are some states which revert it back to February 28; this can be relevant for determining one’s age for voting (18) or consuming alcoholic beverages (21), among other issues. These differences may reflect diverging perspectives on the nature of the leap day.
Lehavdil, the 30th of Adar Rishon creates similar questions. The apparent reading from the Gemara in Niddah (45b) and Arakhin (18b) is that a person’s age changes on their birthday.[8] Rambam (Mishneh Torah Hilkhot Ishut 2:21) writes that whenever the term years is mentioned with regard to the age of a person, it does not mean the lunar or solar year but rather the Jewish year, “whether it is a regular year or a leap year,” indicating that we do not view the type of year as relevant to the calculation. But the Gemara and Rambam do not specify what happens if the relevant day does not exist in the relevant year.
Let us consider a few cases that illustrate the complicated nature of the question. If a person were to die or be born on any other day of Adar Rishon, the yahrtzeit or birthday in a non-leap year would logically be on that same date in the month of Adar (see further below). But what if the person was born or died on 30 Adar Rishon? When should the birthday or yahrtzeit be commemorated in a year which has no such date? Specifically, what is the status of a child born on 30 Adar Rishon whose 13th year for a boy, or 12th year for a girl, is a non-leap year―when is he or she considered a halakhic adult? This question can be critical, as there are serious halakhic implications of adulthood including eligibility for punishments and the ability to partake in certain mitzvot.
Let us consider several possible solutions to this issue that have been discussed by different poskim.[9] R. Jacob Ettlinger, in his responsa Binyan Tzion (1:151), was one of the first to weigh in (albeit, parenthetically) on the question.[10] He notes that Shulhan Arukh (OC 55:10) writes that if there are two babies born in a leap year, one on 29 Adar Rishon and the other on 1 Adar Sheni, and their bar mitzvah year is a non-leap year, the baby born on 29 Adar Rishon will not be considered an adult until the 29th of Adar, while the Adar Sheni baby, despite having been born on a later date, will be considered an adult almost a month earlier on 1 Adar.
R. Ettlinger argues that while it is not explicitly mentioned, the implication is that in our scenario, a baby born on 30 Adar Rishon (the first day of Rosh Hodesh Adar Sheni) would later be considered an adult in a non-leap year already on 30 Shevat (the previous month, which is the first day of Rosh Hodesh Adar) because we look at a two-day Rosh Hodesh as a yoma arikhta, one long day. Therefore, the child born on 30 Adar Rishon can be said to have his birthday on the first day of Rosh Hodesh Adar, which is indeed 30 Shevat in a non-leap year.
Rabbi Yitzchok Yaakov Weiss in his responsa collection Minhat Yitzhak (6:8) is less sure about this inference from Shulhan Arukh and sees the omission of this case as an indication that there are arguments in favor of alternate perspectives. He concludes, however, like Binyan Tzion that the child in our case will become an adult on 30 Shevat, arguing that this exact question was in fact dealt with already by Magen Avraham in the context of yahrtzeit. Indeed, Magen Avraham rules definitively that if a person died on 30 Adar Rishon, the yahrtzeit is commemorated on 30 Shevat in a non-leap year.[11] The same should thus be true for the bar mitzvah case.
R. Natan Gestetner in his responsa Lehorot Natan (13:1) challenges this point, noting that there is a distinction between a bar mitzvah and a yahrtzeit that Magen Avraham himself admits to. While almost every month is presently set on our calendar as to whether it has 29 or 30 days―for reasons that are beyond the scope of this article―the months of Marheshvan and Kislev (while both are usually 30-day months) can have either 29 or 30 days, creating similar problems to the ones being discussed here. Regarding a case where a boy was born on day 30 of Marheshvan or Kislev, and his 13th year lacks such a day, Magen Avraham (55:10) rules that the bar mitzvah should not be until day 1 of the new month because we require 13 full years to pass before considering the child an adult. Regarding a yahrtzeit in this scenario, however, he rules (568:20) that it is commemorated on day 29 of the previous month.
Since we see a difference in terms of the ruling between a yahrtzeit and a bar mitzvah in the Marheshvan/Kislev case, it stands to reason that a similar distinction could be made in the 30 Adar Rishon case, and nothing can necessarily be extrapolated from Magen Avraham’s position regarding a yahrtzeit to the bar mitzvah scenario.[12] It is worth noting that we likewise see a distinction between bar mitzvah and yahrtzeit in the opinion of Rema, who rules that a boy born in a non-leap year marks his bar mitzvah in Adar Sheni if the 13th year is a leap year (OC 55:10). Elsewhere, Rema writes that if a person dies in a non-leap year, the yahrtzeit is commemorated in Adar Rishon (OC 568:7). The reason for the distinction may be that a yahrtzeit is subject to custom and leshon benei adam, the way people speak, which may favor Adar Rishon (see Nedarim 63a) as opposed to bar mitzvah, which is a definitive date/time.
Binyan Tzion and Minhat Yitzhak both cite a responsum of Mahari Mintz to bolster their argument that the bar mitzvah is marked on 30 Shevat. He writes that Adar Rishon is an “extra” month and thus does not have any further effects on the other months. If a baby is born in Nissan in a non-leap year and his 13th year is a leap-year, obviously his bar mitzvah would still be in Nissan and not in (either) Adar. By the same logic, if a baby is born in Adar during a non-leap year and his 13th year is a leap year, his bar mitzvah will be in Adar Sheni, which is the “real” Adar, while Adar Rishon is just an “extra” month.[13]
If we were, hypothetically, to call this extra month not Adar Rishon but by the name of the preceding month, e.g., “Shevat Sheni” or some derivation thereof, nobody would think that it functions as Adar. The uncertainty stems only from the fact that we call it Adar, but we are certainly not going to consider a boy to be an adult for punishment and other purposes simply because of the name we use for the month. In other words, Adar Rishon is really an extra month, having no impact on Adar events. Consequently, the boy born on 30 Adar Rishon, the day before the “real” Adar, would become an adult on 30 Shevat, the day before the “real” Adar in a non-leap year.
In contrast, R. Natan Gestetner disputes the application of Mahari Mintz’s opinion to this question. He notes that we certainly do not view Adar Rishon as another Shevat because if we did, a baby born on 30 Shevat in a non-leap year would not be considered an adult until 30 Adar Rishon (as both are observed as the first day of Rosh Hodesh of the “real” Adar). What Mahari Mintz is stressing is that Shevat is extended, and thus 30 Shevat is really considered part of Shevat, not Adar. Accordingly, a baby born on 30 Shevat in a non-leap year, which is referred to as Rosh Hodesh Adar, will still celebrate his or her bar or bat mitzvah on 30 Shevat in a leap year and will not need to wait until 30 Adar Rishon (which one might think is the true Rosh Hodesh Adar).
But this has no bearing on the case of a boy born on day 30 of Adar Rishon whose 13th year is a non-leap year. Instead, he argues, the correct ruling is that such a boy would need to wait until 1 Nissan to be considered an adult, since we view the two months of Adar as one, as if they were placed on top of each other, and one must thus wait in a non-leap year for all the days of Adar to be complete, just like he would in a leap year.
A somewhat different approach to the question is undertaken by R. Yechiel Zilber in his Beirur Halakhah.[14] He first notes that suggesting that the boy be considered of age on 30 Shevat would not make any sense, since he was born over 30 days after that date―how, then, can one say that he has completed 13 years at that point, on a day corresponding to a date 13 years earlier on which he had not even been born yet.[15] He adds further that the aforementioned Marheshvan/Kislev case, regarding a boy born on day 30 of Marheshvan or Kislev whose 13th year lacks such a day, is not comparable to the Adar case for a significant reason.
As to whether the day is absorbed into the previous month or considered part of the new month, one can reasonably debate the ruling in a situation where there is a day 30 that does not always happen. But in the Adar case, 30 Shevat exists every single year and is certainly not the date on which the child was born; it thus does not make sense for the baby born on 30 Adar to mark his or her bar/bat mitzvah on a date a month earlier. He then suggests that the correct date should be 1 Adar, the reason being that if Adar Sheni is indeed the “real” Adar, then we can view 1 Adar Sheni as the day after his birthday and so in a year lacking an Adar Sheni, 1 Adar takes its place.
Ultimately, however, he concludes otherwise and winds up agreeing with R. Gestetner’s conclusion, albeit for a different reason. He notes that during the time when the moon was sanctified anew each and every month―and hence there was no set determination for how many days Adar would be (see further below)―the child in his 13th year would need to wait and see if 30 Adar would exist at all that year, and we certainly would not automatically assign his birthday to an earlier point. Furthermore, he may also have had to wait and see if the High Court deemed the year a leap year and whether another Adar would be declared in the first place! If our question were to come up during Temple times, then the boy would certainly need to wait until 1 Nissan. Now that we have a set calendar, and lacking any clear direction from the Gemara, we are left with that original position. Thus, the youngster will be considered of age on 1 Nissan.[16]
Each of the different options presented above―as to when a baby born on 30 Adar becomes an adult in the eyes of Halakhah if his 13th year lacks such a day―whether we say 30 Shevat, 1 Adar, or 1 Nissan, is reflecting a different understanding and perspective on how to categorize and understand the 30th day of Adar Rishon. Since this leap day does not exist in most years, poskim must decide whether the day is absorbed into Shevat, into Adar, or into Nissan.[17]
If we take a step back, though, we can question why this situation exists in the first place. Shouldn’t the two months of Adar be identical (thus eliminating the possibility of a day 30 in Adar Rishon)? Now, this question is somewhat predicated on the assumption that we are really doubling Adar in a leap year, but as we have seen, not everyone takes that view of the extra month. Still, this does seem to be an issue, and we can examine how this situation came about. The Gemara in Rosh Ha-Shanah (19b) presents a dispute as to how many days there should be in the added month. The first view is that it should be 30 days;[18] a second view is that it is up to the Beit Din to determine; and a third view is that both Adar months are 29 days.
Testimony is brought in the name of no less than the prophets Haggai, Zekhariah, and Malakhi that the two Adars can both be 30 days, or both 29 days, or the first 29 days and the second 30 days, or vice versa. It is then reported in the name of Rav that in the diaspora, they always assumed that the first month is 30 days and the second is 29, unless and until they got word from messengers from the Beit Din telling them otherwise. The Gemara reports the view of Mar Ukva who says that the month before Nissan is always 29 days, regardless of whether it is a leap year or not. In a second reading of the Gemara, this view of Mar Ukva is rejected, and the final decision would seem to accord with the second view that it is up to the Beit Din to determine how many days are in either Adar. This is indeed the implication of Rambam as well (Hilkhot Kiddush Ha-Hodesh 4:5).
As various Rishonim note, however, despite the Gemara’s apparent conclusion, this was assumed to be true only during the times when the months were calculated and sanctified anew each month. Now that we have a set calendar, though, we follow Mar Ukva―the month before Nissan is always 29 days. The Baalei Ha-Tosafot (ibid) write similarly and add that it is proper to have the extra month, Adar Rishon, as a full 30-day month since it is the one adding on to the year.[19] We might add that by following this opinion, we are also reverting back to the practice cited by Rav that was followed in the diaspora, where the status quo was that the first Adar had 30 days and the second had 29.
In his masterful commentary on Rambam’s Hilkhot Kiddush Ha-Hodesh entitled Shekel Ha-Kodesh, R. Chaim Kanievsky notes another reason why this approach was adopted on the set calendar. Rambam writes there in 18:8 that during the era when the new moon was declared each month by the Beit Din, if the situation developed such that there were no witnesses (e.g., because it was too cloudy), the Beit Din would alternate each month between 29 and 30 days (as noted, this cycle was more or less adopted when the calendar was set). Since Shevat is 30 days, this would mean that Adar Rishon should be 29 days and Adar Sheni, 30 days.
Perhaps, however, the rabbis wanted the month right before Nissan to be 29 days in order that those Jews in the diaspora who lived far away would clearly know when Passover would be, as in any other year. Even though this could change if witnesses came and reported a sighting of the moon, most of the time that prior month was indeed only 29 days.[20] Maintaining it also reinforced the principle that we don’t delay the arrival of Nissan by having the month before it be a full month. Finally, the rabbis further decided that the “extra” Adar, Adar Rishon, should be differentiated from the “true” Adar by means of it having an extra day, and that is how we ended up with the current calendar conundrum.[21]
In the final analysis, then, while it is true that the specific calendar date of 30 Adar Rishon functions somewhat like that of February 29 in that it has no existence during a non-leap year, ultimately, it takes the entire month of Adar Rishon to accomplish the correct functioning of our calendar, and the “extra” day serves to highlight the uniqueness of that month as the “extra” month on the calendar.
[1] The overlap of the Jewish and civil leap year is not a particularly rare phenomenon, having occurred every eight years (or every other civil leap year) since 1976 and having most recently occurred in 2000, 2008, and 2016. Then again, it will not happen again until 2052, making this article perhaps more pertinent. When they do overlap, February 29 essentially always falls during Adar Rishon, at least since the year 1900. I thank Sam Weiss for his assistance with the calendar calculations.
[3] See Ramban in his Hasagot to Rambam’s Sefer Ha-Mitzvot, Positive Mitzvah 153, and Sefer Ha-Hinukh Mitzvah 4.
[4] Rabbi Mordechai Willig once tentatively suggested that the practice of a bar mitzvah boy starting to wear tefillin 30 days before the actual date on which he turns 13, a popular practice without a clear source, may initially have been adopted for “Adar babies” whose bar mitzvah fell out during a leap year, for whom the question of when to begin wearing tefillin, a biblical obligation, was highly relevant.
[5] See https://dinonline.org/2014/03/03/which-is-the-true-adar/ for a straightforward overview.
[6] It is worth noting that while Maimonides considered this entire process to be part of the biblical obligation of sanctifying the new moon, Rabbeinu Hananel claimed that it is the calculation done to assess which month required the extra day that fulfills the commandment while the witnesses were ancillary and served merely to confirm the court’s calculation. See citation and discussion in Rabbeinu Bahya’s commentary to Shemot 12:2.
[7] As an interesting aside, leap years have a (minor) effect on presidential terms in the United States. Since―as per the United States Constitution―every term is four years, it automatically includes one leap year. However, part of the calendar reform of Pope Gregory XIII in 1582 (from which we get the modern Gregorian calendar) was to cut out the leap year on years divisible by 100 unless they were also divisible by 400. The years 1700, 1800, and 1900, therefore, were not leap years. President John Adams was elected in 1796 and his term ended in 1800, while President William McKinley was elected in 1896 and his first term ended in 1900. Since 1800 and 1900 were not leap years, their respective terms were technically a day shorter than any other full presidential terms (excluding George Washington’s first term, which started later than any other due to travel delays, and Franklin Delano Roosevelt’s first term, which was shortened by the 20th Amendment to the Constitution―although this ultimately had no relevance, since he died in the middle of his subsequent fourth term). This issue was not relevant for President Bill Clinton’s second term, as the year 2000 was divisible by four and therefore a leap year. This will only come up again for the presidential election in 2096, i.e., the term ending in 2100.
[8] See Shakh to Hoshen Mishpat, 35:1.
[9] It goes without saying that this article will not cover every source that discusses this question, and it certainly should not be relied upon halakhah le-ma’aseh.
[10] The question he was asked was somewhat different: If a baby was born in a non-leap year on 30 Shevat (observed as the first day of Rosh Hodesh Adar) and his 13th year is a leap year, when is he considered an adult? R. Ettlinger answers that he is considered an adult on 30 Adar Rishon (the first day of Rosh Hodesh Adar Sheni). In the course of his argument, he addresses our case.
[11] This reasoning is cited by Mishnah Berurah in 568:2. It is important to note that there is some question as to the correct text of Magen Avraham, but this article will assume the version as it is quoted by the authorities under discussion.
[12] In a later teshuvah (8:7), Minhat Yitzhak responds to this argument by noting that Magen Avraham’s reasoning for commemorating the yahrtzeit on 30 Shevat is that Adar (in a non-leap year) and Adar Sheni occupy the same calendar space so that 30 Adar Rishon―the first day of Rosh Hodesh Adar Sheni in a leap year—is the same as 30 Shevat, the first day of Rosh Hodesh Adar in a non-leap year. This logic applies equally to bar mitzvah and yahrtzeit as opposed to the reasoning given regarding the Marheshvan/Kislev case, which is quite different.
[13] It is important to mention that, as noted above, there is a dispute as to which month is the real Adar with a minority view maintaining that the second Adar is actually the extra month. See Shu”t Maharil 31. Such a stance would obviously negate Mahari Mintz’s logic.
[14] Vol 1, 166-168.
[15] This argument highlights another matter of dispute that would complicate matters further. While most poskim assume that the person’s age changes at the beginning of the birthday, there are some who maintain that we must wait for the specific moment of the day when the child was originally born before considering the age to have changed. See the comments of Netziv in his Ha’amek She’elah to Sheiltot 116. Calculating that moment in time in the case under discussion here would be very complex.
[16] R. Zilber has a longer piece analyzing the yahrtzeit question as well. It should be noted that the Dirshu Mishnah Berurah quotes Rav Yosef Shalom Elyashiv and Rav Chaim Kanievsky as ruling that the boy must wait till 1 Nissan until he is considered a halakhic adult.
[17] A somewhat similar problem exists in the laws of niddah. While a woman is considered to be in niddah only with the appearance of menstrual blood, there is an additional concern, which created the halakhic category colloquially known as vestot―days or periods of time where there exists the possibility, or likelihood, that a woman may start to bleed and thus intercourse is forbidden. Most of the vestot are based on an assumption of regularity/predictability of the woman’s cycle. One such type is known as veset ha-hodesh, whereby the calendar date when a woman starts to bleed in one month dictates that she must refrain from intercourse on that same calendar date of the following month, even if she has not yet seen any blood. The uncertainty arises when a woman bleeds on day 30 of one month and the next month does not have a day 30. This is a major question without a clear resolution from poskim. As an aside, it has always struck me as odd that this question does not really seem to have been dealt with until the 19th century, when it certainly must have come up in earlier generations.
[18] Interestingly, Turei Even explains that according to this view, the rabbis set it this way so that messengers would not need to go out and alert people about Adar Sheni, since it would already be known.
[19] See Arukh Ha-Shulhan He-Atid, Shonim 99:6-7, who argues for a similar understanding within Rambam.
[20] Similarly, R. David b. Zimra (Radbaz) in his responsa (1:150) suggests that the while the first Adar is 30 days because it is the extra month and not common, the second Adar was set as 29 days in order to not confuse those who know that there are always 30 days between Purim and Passover and thus avoid the possibility that they may come to miscalculate the days of the holiday and eat hametz on Passover.
[21] It is worth noting that, as Rambam writes, the Beit Din always has the right to use the calculation system to determine when the month begins independent of the presence or absence of witness testimony. Perhaps it was set that whenever witnesses did not appear, they would reverse the usual sequence, such that Adar Rishon had 30 days and Adar Sheni had 29 days, specifically for the sake of the diaspora Jews, so that they could rely on the standard calculation based on the majority of years and therefore know when Passover would fall out. This common situation was then incorporated into the set calendar, despite whatever problems it created.
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