Date in a dropbox box

Hi there everyone, I have a form which displays 3 dropdowns, day, month and year, I have it displaying the default of the dropdown for month no problem, but how can I get it to select the current date (day) in the dropdown, but show 31 days nomatter how many days are in the month? This is important even though it sounds odd :-) so if it was the 5th of a month, it would show 5 as a default value, but allow you to select 1-4 before that and 6-31 after it. Thanks everyone Chris