I never got an answer to what might have caused the problem. I later upgraded to MS Office 2015 for Mac and the exact same formula is working again.

I have a spreadsheet that capture a datediff using a macro. when I ran the macro in previous version it gave me only difference in years. now the exact same spreadsheet gives me the correct difference in days. so I have this inconsistency in different rows but I didn't bother to fix it.

so my conclusion is that the datediff function is not compatible in a previous version of office.