if time_t is unsigned and the current time is less than uptime.

There's comments that suggest the time(NULL) value is influenced by the
timezone, so possibly therre's a usage of it somewhere with the timezone
not being set comparing to an uptime that was set when timezone *was* set.
In this case, @UPTIME@ will say it's been up for zero days until the timezone
is set in that thread/process, or time(NULL) passes uptime.