On time and how much has elapsed
http://office.microsoft.com/en-us/access-help/on-time-and-how-much-has-elapsed-HA001110218.aspx
Dates and times play a vital role in most database applications. In this article, learn how Access stores Date/Time values and how to calculate elapsed time, such as might be needed in a timesheet application. We provide a downloadable sample database with three sample functions you can copy and paste.