I have a formula DateTimeDiff([effectiveEndDate],'2021-03-31',"Month") that is working sometimes but not always. Someone else helped me with my workflow and I'm not very familiar with how date time diff works. You can see below in my sample that it works sometimes but not always. Most of the ones that aren't working are 1 month off. What do I need to do to fix this?
Incorrect:

Correct:

Thank you in advance!