Found the answer. You have to create a field and use the following formula as the default value:
Substring(datefield,6,2) to get the month from a date field.
Then create another field and use the following formula as the default value:
Substring(Today(),6,2)
Then, on the date field, add a validation rule stating field A must not be equal to field B.
This will set the date field to accept only dates from the current month as today.
Go me!