[tz] source code question regarding localtime.c

In localtime.c, function localsub() are these three lines of code: 1295 icycles = tcycles; 1296 if (tcycles - icycles >= 1 || icycles - tcycles >= 1) 1297 return NULL; I do not understand the reason why lines 1296 and 1297 exist. icycles and tcycles are equal. Their difference is always zero. Why test for it being not zero?