[AccessD] Forcing a date calculation

All,

I have a form that users are supposed to enter a start date, always a Monday
and an end date, always the Sunday after.

However, they seem completely unable to enter these dates correctly so I am
trying to see if there is a way to force a date into the system using Date()
and then going back to the previous Sunday and then working out the Monday
prior to that.

So, 31 August 2005 would go back to Sunday 28th August 2005 and I would then
subtract days to get Monday 22 August 2005.

Could anybody give me a pointer on if this is possible as the data being
returned currently is normally incorrect and this is causing more than a few
headaches !!

Thanks in advance,

Chris Swann