The oldest clock and calendar is so called Bible calendar. Each day has 24 hours divided in 2 parts of 12. Each year is always the same: 360 days or 30days in each of 12 months. It needs just one adjustment over forty years or two in our average lifespan and do not start or finish at 0. Unfortunatelly, our clocks and computer clocks must be adjusted for different world time zones more often. For this purpose you can use Greenwich Mean Time(GMT) and Network Tme Protocol(NTP).