The Julian date is the number of days that have passed since noon on January 1st, 4713 B.C. Julian days begin and end at noon, instead of midnight. This allows an entire star viewing session to be in the same Julian day.
Note: Noon is actually 12:00 PM. Midnight is 12:00 AM on the day that is just starting. For example, the midnight between January 1st and January 2nd is actually 12:00 AM January 2nd.