What problem does this solve or what need does it fill?

(This issue discusses a post-#7267 Bevy: follow that PR for context.)

Migrating systems between CoreSet::Update and CoreSchedule::FixedUpdate should be painless.
In particular, Time and FixedTime should be one-for-one drop in replacements for each other.

What solution would you like?

Fully synchronize the methods available between Time and FixedTime.
Every method on one should be represented on the other wherever possible.

This can probably just be done manually to start.

What alternative(s) have you considered?

We could use a trait here, but I'm not sure the indirection / complexity is worth it.