Add tests revealing truncate's inability to properly handle weeks due to datum consistency issues.
Avoid context-enabled conversions during truncation for like terms.

This fixes week truncation as context conversion tripped an off-by-one issue.

I suspect there is still some problems in this general area per the impossibility to express
Date(0) in weeks. Whatever the case is, working with Week based dates is a bad idea when
considering the gregorian calendar.