Here is a simple program that fails.
int time1 = clock(); // seconds count
int time2 = clock();
if (time2 - time1 > 30) call change_traffic_light_color();

At 3:14:08 Jan 19 2038, time2 will be a negative number and time1 a positive number for the first time in 68 years. The traffic light color will stop changing.