Julian date in days since noon November 24, 4714 BCE:

Julian date variants have different epoch dates:

Convert from a date object to a Julian date:

Convert from a date string to a Julian date:

Inputs are assumed to be in $TimeZone. Block can be used to specify a different input timezone:

Use DateObject to specify an input in a different TimeZone:

Separators are assumed if not specified:

Give explicit separators:

Date lists are converted to standard normalized form:

Day, hour, minute, and second values in input date lists can be noninteger:

The current year is used if not specified in a string:

Standard Julian dates have zero fractional part at GMT noon in the UTC time system:

Therefore they have fractional part 0.5 at midnight:

Modern Julian dates are large numbers, almost two and a half million:

More than machine precision is needed to keep track of the computer's clock accuracy in days:

This is the accuracy in seconds:

It coincides with the accuracy of other date representations denoting seconds with a real number:

JulianDate[date] converts date into Julian date format:

FromJulianDate converts from Julian date format into a DateObject expression:

Julian date 0 corresponds to this date in the proleptic Gregorian calendar for the GMT time zone:

It corresponds to noon January 1, in the Julian calendar for the GMT time zone:

Convert back to Julian date 0:

JulianDate is an affine transform of AbsoluteTime: rescale from seconds to days and add a constant:

Take any date:

Then this is an alternative way to compute Julian dates: