Excel Using Dates

Essentially, all I'm trying to do is check if the cell is empty, then return an empty cell, otherwise, i want to return a date that is 21 days ahead of the date in the cell. Here is what I have so far: =IF(ISBLANK(A:A), "", (DATE(YEAR(A:A),MONTH(A:A),DAY(A:A)+21)))

However, for and example cell in col A: 12/18/2015, this function will return 42377