It's both location and time-since-last-use. They download the parameters for a mathematical model called an almanac which they use for a first-guess location of the satellites. The satellites drift away from where the almanac says they are, so it becomes less useful over time and effectively expires after about 3 months. Downloading a new one can take half an hour or so; it's transmitted by all the GPS satellites, but slowly. So if a GPS unit hasn't been used for over 3 months, it can take a while to get fixes until it's held a fix for half an hour or so.

There is also another model called the ephemeris, which is much more precise and expires after a few hours. That's the one it actually uses to fix your location. It expires after a few hours, which is why a "warm boot" is faster than a cold one.