Rework to_batch_interval_start for the new Time world

[jcjones]

This is just not ergonomic. I did it this way because to_batch_interval_start() is just odd to reason through hanging off of DateTimeExt -- for this purpose anyway. It's also a bit weird to hang off Time because then we need basically two time_precision arguments.

I'm going to open a follow-on to tackle this better. For now, this may be dumb but it works.

Originally posted by @jcjones in #4219