What's the purpose of SetDate?

SOLVED

I took the BOI class, but that seems to be intended as a high-level overview, not so much a function-by-function breakdown. I checked the documentation, but it just says "Set module date". Not exactly helpful.

So, what's the effect of setting the module date? I've played around a bit, changing it to several different values, and it doesn't seem to have any effect when I'm querying tables. Is it used more when writing data out to objects, or... what?