Well, you could add some data validation for this, I suppose - check that the years and months match (using substring functions to get the separate pieces of the date) - but it won't prevent the user from being able to pick a different date - just prevents him / her from submitting. You could also reset the field to blank if the date selected wasn't in the same month / year, and display a message about the need to select a date in the same month / year.
There's a sample form here with some info on getting parts of date fields with substring functions.