Rule:
Condition: substring(today(), 6, 2) = 1
Year = substring(today(), 1, 4) - 1
Month = 12
Condition: substring(today(), 6, 2) != 1
Year = substring(today(), 1, 4)
Month = substring(today(), 6, 2) - 1
Or without conditions:
Year = substring(today(), 1, 4) - (substring(today(), 6, 2) = 1)
Month = substring(today(), 6, 2) - 1 + 12 * (substring(today(), 6, 2) = 1)