Julian day
The Julian date (JD) is a continuous count of days and fractions elapsed since the same initial epoch. Currently the JD is . The integral part (its floor) gives the Julian day number. The fractional part gives the time of day since noon UT as a decimal fraction of one day or fractional day, with 0.5 representing midnight UT. Typically, a 64-bit floating point (double precision) variable can represent an epoch expressed as a Julian date to about 1 millisecond precision.