Abib vs. Today's Calendar
The dehioth (postponements), seen in the Jewish calendar and implemented by Hillel II (2nd) around 359 C.E. to keep Sabbaths from falling back to back, is not justified by Scripture. In this case, and many others, the end most certainly does not justify the means.
The following Scripture comes to mind when thinking of what the Pharisees did in the past:
They tie up heavy burdens and lay them on men's shoulders…, Matt 23:4.
The confusion and burden in the numbers is also evident by looking at the short history of the Julian into the Gregorian calendar of 1852, which ended up making a 21 day month in October of 1852.
'Julian and Gregorian Calendars. Sweeping reforms in the Roman calendar were instituted by Julius Caesar in 44 b.c. with the help of Sosigenes, a Greek scholar who based his solar calendar on a year of 365¼ days. Although the radically transformed calendar ended much confusion, its year was about 11 minutes longer than the true solar year. Since the Julian calendar was used for more than 15 centuries, that tiny error eventually produced gross discrepancies. Bede, an 8th-century English monk and historian, and Roger Bacon, a 13th-century English Franciscan friar and scientist, were among those who reported on the problems of the Julian calendar.
By a.d. 1580 the equinoxes were falling 10 days earlier in the year than they had in 44 b.c. 'By 1582 the errors of the Julian calendar were deemed too great to tolerate; in that year Pope Gregory XIII issued a decree abolishing the ancient calendar. In its place he substituted what is commonly known as the Gregorian calendar. At first, few countries beyond the influence of the papacy accepted the new calendar. Some resisted the changeover for centuries. Britain, for example, refused to adopt the Gregorian calendar until 1752, Russia until 1918, and Turkey until 1928.
'Gregory's edict took effect in October 1582. In that initial month the 5th day was designated the 15th, thereby compensating for the loss of the approximately 10 days from the Julian calendar. Simply stated, the Gregorian rule declared that a year whose number is exactly divisible by four would be a leap year, with the exception of century years (such as 1700,
1800, 1900, etc.) unless they were an exact multiple of 400 (such as 1600, 2000, etc.). In each leap year February would have 29 days instead of its normal 28. February in the Julian calendar had been shortened from 29 to 28 days by the emperor Augustus in order to add a day to his month (August)—to make it the same length as the month named for Julius Caesar (July)! The Gregorian calendar left the lengths of the months the same as in the Julian (as revised by Augustus).' –Baker encyclopedia of the Bible. Elwell, W. A., & Beitzel, B. J. (1988), page 400.
All these changes, all these years, again and again. It is clearly shown, as quoted at the beginning, 'Devising a calendar that works has thus been one of mankind's longest quests.'