If I understand correctly, you are interested in just the month part of the date .
In that case, an option would be to extract the month from the date by using the “Get Integer From Datetime” Java action included in Community Commons.
From there you could build the logic stating 1 = January, 2 = February, etc
To add to Ruben's answer:
If you can't select a date from the dropdown, use the “edit” button to select another object, like this:
So you could write $YourEntity/Date or something similar to get the date in question.