如何减去两个日期和时间来得到不同?

i have to sent an email when a user register email contain a link that is become invalid after six hours what i m doing when email is sent i update the db with field emailSentDate of type "datetime" now i got the curent date and time and has made to the same formate as it is in db now i want to find that both these dates and time have differenc of 6 hours or not so that i can make link invalid but i donot know how to do thisi have to sent an email when a user register em