i followed steps on this link to add a date field to my custom module :
http://magentomechanic.blogspot.com/2010/01/to-add-custom-date-field-in-custom.html
Every
print_r() your data in controller and exit before saving it and look if you are still getting the right data before saving it. I never had this issue but it seems to be issue of your time zone.
print_r()