The hours calculator calculates the difference between two hours. It is also capable to calculate difference between two hours which are in different days.
Ancient Greeks divided daylight time into 12 hours. Later they started to divide night into 12 hours too. It is believed that Hipparchus of Nicaea is the first person to divide day-and-night into 24 hours. Thus one hour can be defined as the 1/24 of one day-and-night time.