A calendar notation in which the date is represented by one number. For example, the Julian date for December 11, 1942 is 2430705; while December 12, 1942 is 2430706.

The Julian calender dates back to 45 BC, when the solar year first was fixed at 365 days long plus one extra day every four years. But that calculation still is 11 minutes off, and over the years ...