Need help creating user exit variables

Hello friends,

Please help me with steps on how to create user exit variables on 0CALDAY in the following scenarios.

1. Create new variable -- get default -- low -- first day
   of previous week , high -- last day of previous week.
2. Based on cal week -- range variable -- derive -- the
   low as first day of low cal week range and high as last day of cal week range.
3. Based on cal month -- range variable -- derive -- the
   low as first day of low cal month range and high as last day of cal month
   range.
4. Based on cal quarter -- range variable -- derive --
   the low as first day of low cal quarter range and high as last day of cal
   quarter range.

Greatly appreciate your help!

-Isaac