The DATE function in DAX has this simple syntax:
DATE( <year>, <month>, <day> )
If you are like me, you never read the BOL notes that says in a clear way that it supports dates beginning with March 1, 1900. In fact, I was wrongly assuming that it would have supported any date that can be represented in a Date data type in Data Models, so all the dates beginning with January 1, 1900. The funny thing is that in some of the BOL documentation you will find that Date data type supports dates after March 1, 1900 (which seems not including that date, but this is a detail…). But we should not digress. The real issue is that if you try to call the DATE function passing values between January 1 and February 28, 1900, you will see a different day as a result.
evaluaterow ( "x", DATE( 1900, 1, 1 ) )
-- return WRONG result
-- [x] 12/31/1899 12:00:00 AM
evaluaterow ( "x", DATE( 1901, 2, 29 ) )
-- return WRONG result
-- [x] 2/28/1900 12:00:00 AM
evaluaterow ( "x", DATE( 1900, 3, 1 ) )
-- return CORRECT result
-- [x] 3/1/1900 12:00:00 AM
As usual, this is not a bug. It is “by design”. The DATE function works in this way in Excel. And also in Excel it was “by design”. In this case the design is having the same bug of Lotus 1-2-3 that handled 1900 a leap year, even though it isn’t. The first release of Lotus 1-2-3 is dated 1983. I hope many of my readers are younger than that. I tried to open a bug in Connect. Please vote it. I would like if Microsoft changed this type of items from “by design” (as we can expect) to “by genetic disease”. Or by “historical respect”, in order to be more politically correct. 