I imagine that there's a simple counter for daily usage, and that this can be modified, but I've been unable to find any documentation on where or how to do this. Does anyone know?
appears to contain a key/integer pair "duration" that entirely controls the remaining time for the current day.
The syntax is quite easy to understand, although how the system react to changes in the file is not quite clear. For instance, the timer in the username menu breaks if edited while the user is logged in. Everything does appear to get reloaded on login, however, so it should be easy to extend the time remotely via shell once it has run out. Beware, the time format is wonky, though. I've only managed to test three digit numbers for duration, and they appear to be interpreted as Minute Second Second.
EDIT> The above theories are not correct. It appears that the value is actually a value in seconds, but it must be added to some expirytime that is not in this file, but probably in the individual binary plist files stored under the same hierarchy in year/month/day, but it is very hard to decipher these.