 For compatibility with Lotus 1-2-3, the 1900 Date System incorrectly accepts February 29, 1900, however, 1900 was not a leap year.While most software, including Excel and R, recognize 4000 and 8000 as leap years (as they are divisible by 400), SAS does not (an unofficial "4000 year rule").Thus date conversions between SAS and other software go out of sync after February 28, 4000, without accounting for that discrepancy.