Shrug. The algo given relies on three things:

1. The number given by time() increases by 1 every second of time that passes (IE is in seconds since some epoch.) (This is true for all OSes, and is part of the defintion of time() given in perlfunc.)
2. The number given is invariant on the TZ the computer is in, assuming the computer is set up correctly. (This is true for all current ports that I am directly aware of, but not gaurnteed by perlfunc:time, nor perlport, as far as I can see. (I suspect it's even true for DOS, if you set $ENV{TZ} correctly, which I include under "assuming the computer is set up correctly", but I see no documentation of that in perldos.))
3. The epoch refered to in point 1 is on midnight of some day. This isn't gaurneed by perlfunc:time, nor perlport, but I'm unaware of any platform where it isn't true, and it seems quite insane for it to be untrue.

It should probably be noted that the only platforms I have programming experince on are DOS-based or Linux.