Actually not that bad. If everyone assumed today's date to be the max value, one could always add something new on top without having to guess whatever value someone else has used, and risk exhausting the range because someone uses the max value.
It's not half bad, the problem is that you'd have to throw an error when attempting to use higher values (or in CSS's case just not render it or something), because currently z-index will happily accept >max values and just consider them max.
But what's today for me is yesterday in New Zealand and if I'm on the West Coast of the US or Hawaii then that can go on for a 'long time'? If I then go +1 then that's just going to end up in the same rat race / breakages.
If CSS would up it to be a 64 bit integer then timestamp could fix all that. That's a common approach to DNS serials, there they are 32 bit but there is logic about handling rollovers that wouldn't apply to layers. Or if you want to keep it human readable UTC.
