code to make a calender auto update

im wondering how you code the text boxes once you know the base portion of a month to update the other days. also seeing a 31 vs 30 problem but thats not a huge issue here.  i can update this manually but its geting to be  a pain and clearly some coding exists for *variable* plus one , plus 2 etc. planning this in as3 coding but also could do in html5 as well since i also got a project id like to do where all the files are numbered 1 to 1000ish for both the thumbnails and the swf.