What's a good wat to trunctate a time point

[Dukc via Digitalmars-d-learn]

I have a time point, be it SysTime or DateTime, whatever. I want 
to trunctate it to weeks, eg. I want it to become the first point 
of time during the week it was representing. What's a good way to 
do that? Only hacks came to my mind.

The solution does not have to be generic, trough I'd prefer it to 
be if it can be without much extra work.