Use a calculation (expression) to subtract 12 months from either the current date or some provided date - whichever best suits your need. This should calculate a date 12 months prior to today
dateadd("m",-12,date)
Perhaps take a look at DateAdd function?
EDIT - forgot to say your post isn't clear. 12 months prior to today, or from the 1st of the month that is 12 months ago? Probably the latter. Perhaps post your form sql.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.