I'd use 2 parameters, {?Year} and {?MonthRange}.
{?Year}
Value type: Number
Options: Discrete values
{?MonthRange}
Value type: Number
Options: Range values
**Optional, but nice for users**
Hit 'Set default values' and enter the month values (1, 2, 3, etc.), then add the Description for each (January for 1, February for 2, etc.). Set 'Display' to Description, 'Order' to Numerical ascending, and 'Order based on' to Value.
Your record selection formula would look like:
(Year({table.DateTimeField}) = {?Year})
and
(Month({table.DateTimeField}) in {?MonthRange})
**Note - this may not pass through to the server. I can suggest a way around this, but it's database dependant.
-dave