#include <time.h>
time(NULL) // returns the time as the number of seconds
           // since the Epoch, 1970-01-01 00:00:00 +0000

Year 2038 problem …

• time(NULL) on 19 January 2038 at 03:14:07 (UTC) will be 2147483647 = 0x7FFFFFFF
• a second later it will be 0x80000000 = -2,147,483,648
• ⇒ -2³¹ seconds since 01/01/1970 ("Epoch") is 13 December 1901 …