You need to look up the fiscal month ending date that you have stored in your accounting months table.
In this example, ACCTGMOS_TABLE is my table, and I need Report_Date (a field on my form) to fall between DT_ACCTNG_MO_START and DT_ACCTNG_MO_END. If it does, I pick up DT_ACCTNG_MO_END.
Something like this:
=DLookUp("[DT_ACCTNG_MO_END]","ACCTGMOS_TABLE","([Report_date] >= [DT_ACCTNG_MO_START] ) And ( [Report_date] <= [ORAD_ACCTGMOS_T1]![DT_ACCTNG_MO_END]) ")