When calling Std.Date.AdjustCalendar(); with February as the MM portion of the date, i.e. '20170201' and -13 as the month delta, Std.Date.AdjustCalendar(); incorrectly returns December 31st 2 years prior, so '20151231' for the above example. It will return December 31st 2 years prior no matter what the DD portion of the date is, i.e. '20170228' will return '20151231' also. This error happens for every YYYY as well. For example, '20160201' will return '20141231'.