You could try something like this in your script:
(Month(Today())- (right(Monthfield,1))) as Month,
This will return number of the right month.
But you have to modify it to your own script, but maybe you can start from here.
And keep in mind that you could better solve it on a MonthYear field in stead a MonthField.
Unfortunately that didn't work. But I've managed to get a number output by writing;
=Month(today())-Month(30) This gives a result of 6 (Jun)
=Month(today())-Month(60) This gives a result of 5 (May)
=Month(today())-Month(90) This gives a result of 4 (Apr)
and so on ...
If I could turn these numbers into month names that would solve my problem I guess..
I would not use "month(today()) - month(30)" to get the previous month, but month(today())-1
Both result in 6 (as of today), buth month(30) means month('1900-01-29') which is Jan (or 1), but I think this is misleading and depending on start of internal calendar.
In above expression, I make a date (because month() expects a date, using calculated month number and arbitrary year and day number (i.e. 2011, 1)