What about making status effects that are based on the time system, and shut on and off based on what day, week, or month it is.

Example ("That time of the month" when all female wizards lose their powers)

But you could also have some curse happen at the last of every 30 days or so (or full moon). Usually, these status effects should go away by the next day or week, since other stuff shouldn't cure them (inns should be custom here). What do you think?

Always a good way to vary up your game. Players will need to be more aware of the time system and how to manipulate it to their advantage.

Always remember these systems play a minor role (to reduce frustration from the controller). If you over-emphasize it you may have to remove extra innovative systems to keep players from looking all over the place for answers to why their spells aren't working like they should. Also from a designers stand point you don't want too many variables in place creating wild and unpredictable results. Good design begins with predictable results from the developers point of view.

Over-all, it seems like a pretty good idea. It would operate similarly to the Elemental system, in that it's a puzzle type advantage and disadvantage system. Extending it to affect buffs on enemies or encounters in general would also give it a lot of flavor.

This seems like an interesting idea. You could very much add a custom touch with this. But as said earlier, you need to make it minor, but visable, so the player is aware, but also doesn't have to worry about every living moment.

I've played games where damage from certian classes (Scholar, being a popular one) where they used the ability "Horology" which was based on the factor of time. It was rather weak the majority of the time, but during the night to dawn times--which was like 15 minutes game time--was powerful. This adds an element of "Oh hey, I've got this awesome spell I can use but at certian times, looks like I'm gonna be aware of time to make this more efficient."

Maybe in the "that time of the month" example have a giant female symbol floating over their heads in battle...?

They needn't all be negative as well, Werewolf status would be very positive (aside from the auto attack effect).