The interval scale is a term taken from statistics that is one of various methods of classifying variables. It allows for quantifying the degree of difference between items, but does not measure the ratio of difference between items. In everyday terms this is the type of differences that are measured by thermometers or calendars.
For instance, the Fahrenheit and Celsius scales of temperature are calibrated based on the freezing point of water at sea level but 40 degrees on either scale does not mean that it is twice as warm as 20 degrees. Likewise, in most modern calendars years are counted, either forward or backwards, from a culturally significant event, with months that vary in length.