There's no way 999:23:59 is an smf limit
Or at least, not in the way people are thinking
Since computers store numbers in binary, any number that is perfectly round in decimal would be an arbitrary limit specifically placed by the developers. Meaning it wouldn't be something that they'd have to work more to allow to happen and just didn't do the work because they didn't think it would happen, but rather the complete opposite: they would have to specifically work to add that limit.
So it's many, many times more likely that Idle just logged off before that time to make you all upset.
The length of time logged in would be stored as 32 bit integer (at least, as 16 bit is simply too small)
Which would give a limit of ~49710 days if the time is stored as seconds (more likely, IMO) or 2982600 days if stored as minutes