This would bring in the issue of time zones.
Right now it's a simple fact that every player is impacted by this. I know that during my working day I won't have time to play except for my lunch break, and also during nighttime of course.
My working hours could be someone else's sleeping hours, vice versa, or my off time could be exactly the moment my opponent plans their orders.
As for weekend: when does Saturday start? Should you use US time, GMT time?
I think off time could only be implemented if you can also filter games based on timezone (e.g. play only with players in Wester Europe, etc.). Which is a feature I'm mildly interested in, because it would be fun to play with people who have the same day schedule, but it sounds like a hassle to implement.