From type/DateTime

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method day-fraction(DateTime:D: --> Real:D)

Returns the instant's time as a fraction of a 24-hour day.

say DateTime.new('2021-12-24T12:23:00.43Z').day-fraction; # OUTPUT: «0.5159772␤»

Notice the day-fraction value is the same as the fractional part of the modified-julian-date for the same instant.