With no other references (that I know of) why wouldn't they?
Here's what's going on.
"Party of One" was aired on April 29th. This episode was aired Febuaray 18th and Pinkie Pie said her birthday was 75-days away. This estimation hits May 3rd.
This is obviously a problem. 1) "Party of One" had two days in it, if we accept the theory that their timeline is the same as ours, then her birthday would be on the 30th. 2) This makes it 4-days off (3-days off if 30th).
So her birthday is not only April 29th, but also April 30th and May 3rd. The theory is incorrect.