Astronomers frequently work with events with great regularity in time, and need to be able to calculate the difference between two dates or when a particular phenomenon will recur in the future without checking off boxes on a calendar. The Julian day system meets these needs. A Julian date is simply the number of days elapsed since the start of the year -4712. This avoided the inconvenience of having an observing session begin one day and end the next, as would occur were regular civil dates used. By starting so far in the past, all historical observations can be expressed by positive Julian Day numbers.
Time is expressed as a decimal fraction of a day.
For Julian dates for years before 1 A.D., note that astronomers and historians use different conventions for those years.