From type/Dateish

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method daycount(Dateish:D: --> Int:D)

Returns the number of days from the epoch Nov. 17, 1858, to the day of the invocant. The daycount returned by this method is the integral part of the Modified Julian Day (MJD) which is used routinely by astronomers, geodesists, scientists, and others. The MJD convention is designed to facilitate simplified chronological calculations. The fractional part of the MJD consists of the hours, minutes, and seconds of the using DateTime object converted to the equivalent fraction of 24 hours. Those two values added define the MJD of that instant.

say Date.new('1995-09-27').daycount;    # OUTPUT: «49987␤»