Ok, so here's a shot at this...
As far as the dreamhost form mail goes I believe that the is no way for you to change the time on this. Becuase this is a script that dreamhost has, and you don't have the ability to change it you'll have to deal with the time it sends out.
A way around this would be to install you own form mail script. I heavn't done this recently, but basically you download your CGI script and possibly some associated files, configure it to work on your server. Then you set up your form to post to your script instead of DH's.
I can't reccomend a specific program becuase I heavn't done this in a while. Try searching for it, and looking on sourceforge. Then once you find this program it may have instructions on how to change the time, or you can seek some specific advice from the creator or basic CGI commands. All in all there is not a very simply solution, and you might just want to put up with the three hour difference.
As far as PHP goes, you should also be able to look into you coding and find a way to modify the way it is printing the date. Check out this link
hope this is helpful, sorry if it's not.