Ai(isday) accept an optional time

It would be nice if ai(isday) would accept an additional, optional parameter which allows to specify the date/time which to test. When missing, it would default to now.

This would allow to make checks for dates in the future, whether that will be day or night. Especially kinda interesting when considering weather icons, as it would allow to do something like "$ai(isday, dp() + 6*60*60)" to test whether one should use a day or night icon in six hours.