When we need to make calculations based on time its much easier to use numbers instead of the conventional "Gregorian Calender" that we use today. Imagine trying to calculate the exact number of days between 0h UT on October 25th, 1983 and 0h UT on September 4th, 2007. It can be done, but it is much easier to change the days into numbers and subtract. So, if we change the days to the JD numbers, 2445632.5 and 2454347.5 we see there are exactly 8715 days between the dates. The Julian calendar was invented by the Roman Empire and is a continuous number of days since January 1st in the year -4712. Click on the chalkboard link below to see how its calculated (from J. Meeus).

Astronomers use HJD, or Heliocentric Julian Date to correct for the time difference between the target star and the Earth. Example: If two times of minima for an eclipsing binary are determined to be 2452878.91042 JD and 2452956.95208 JD, roughly 3 months later, then the earth has advanced nearly 1/4 of the way through its orbit. This means, depending on where the star is in relation to the plane of our orbit around the sun, the light from the star may arrive earlier or later than expected. To correct for this we center our times on the position of the sun instead. Click on the chalkboard link below to see how its calculated (from Hendon and Kaitchuck).