gameboy, please elaborate on how you would want such a mechanism to work. Look at the structure of the .xml files and see if you can figure out a place that it would make sense to insert such a thing. The first thought I have is on the upgrades, rather the upgrade is limited in when it is effective. Also, consider that you may want to have 4 points, dawn, day, dusk night, or even 8 points: dawn, morning, noon, afternoon, evening/dusk, night, midnight, early morning/pre-dawn. You may want to have the multiplier for the effect potency be at zero until a certain time when it begins to increase towards 1, then once that time is reached, it gradually decreases until it reaches zero again at a certain time. Or maybe you want it to be zero until 8 PM, when it gradually increases until it reaches one at midnight, upon which it remains one until 4 am, where upon it gradually decreases until it reaches zero at 8 AM. There are many possibilities.
The more I think about it, it would probably work nicely as a constraint to an upgrade/effect/emanation (they all use the same base XML & data structures).