From type/DateTime

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method modified-julian-date(DateTime:D: --> Real:D)

Returns the Julian date (MJD) for the UTC date and time.

say DateTime.new('2021-12-24T12:23:00.43Z').modified-julian-date; # OUTPUT: «59572.5159772␤»

Notice the fractional part of the modified-julian-date is same value as the day-fraction for the same instant. Likewise, the integral part of the MJD is the same value as the daycount for the same instant since they reference the same epoch (November 17, 1858). The MJD is obtained by subtracting the constant 2_400_000.5 from the Julian Date and is used to simplify transformations between civil and astronomical time systems.