When you insert a date or time the local date / time zone of the server is used so for example if your server is in a region where the time is PST then that is the time zone used.
You can override this by placing this statement at the top of your page:
/*
Set Timezone
----------------------------------------------------------
*/
ini_set('date.timezone', 'US/Eastern');
You can find a list of all the supported timezones here: timezones.php
Hope that helps