Access has a number of built in date and time functions. Figuring out what each does and how to use them to get specific results is often difficult. Here are some links to various resources, mostly at Microsoft web sites, that will help in performing some common date time functions in Access. Many of the links contain code you can copy and paste into your applications.
You may find the following links helpful when dealing with dates in Access.
FAQ181-261 - Calculate working days between two dates FAQ705-3213 - Calculating a future Workday Date FAQ181-1374 - Select birthdays within the next 30 days ... FAQ701-3103 - How to determine the First/Last day of a week
Access 20002 (Fewer resources but the Access 2000 code should be apllicable in most cases.)