Time variables are subject to overflow. So, the comparison statement as follow will return incorrect result if t1 has overflowed and t2 has not overflowed.
unsigned long t1, t2; if (t1 > t2) printf("t1 > t2");
The code as follow can be used to deal with the above issue.
unsigned long t1, t2; if ((long)(t1 - t2) > 0) printf("t1 > t2");
Suppose sizeof(unsigned long) = 4, and t1 < t2, the result of t1 - t2 under the unsigned semantics is (0x100000000 + t1 - t2), because of borrow, if the result < 0x80000000, in signed semantics and complemental code, the result is positive.