>I'm resetting a filter with a date literal and it suddenly became... AMBIGUOUS!

I'd suggest using a CTOD() on an index and then filtering on that. There's another reason too, sometimes doing a seek on a datetime will fail, and you don't know why. Its because the DateTime actually store the millisecond in the table, but its not possible to specify the millisecond in a variable, so the seek rarely finds a match, even though one should exist. CTOT() solves that.

By the way, I liked Larry's suggestion, its pretty sneaky!