[twic]

One approach might be to have many instances, like we do with Discord etc, and have the admins choose a timezone, so an instance for French people would be on Paris time, etc.

Or even just choose the start of the time range directly. French joggers might prefer a different time to French Counter-Strike players.

[notpushkin]

A complete opposite approach: same instance for everybody, but you can choose the hours when you sign up. Changing is allowed once every 4 weeks or something.

[lifeformed]

Another idea is that the window drifts slowly, so everyone gets an optimal time. Make the period non divisible by 7 so it doesn't line up on the same day of week every time.