> To be fair, the new opt-in "use strict" here is "switch to Temporal".

This. Don't break old code, just provide new best practices.

Update linters (or ideally first class language rules, like in Rust's "edition"s), to gradually kill off old behavior. Without having to do a decade long Python 2 -> 3 migration.

Temporal is nice. It learned from the many failures and dead bodies that came before it. And it had lots of good implementations to look at: Joda Time, Chrono, etc.