From type/Dateish

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method days-in-month(Dateish:D: --> Int:D)

Returns the number of days in the month represented by the Dateish object:

say Date.new("2016-01-02").days-in-month;                # OUTPUT: «31␤» 
say DateTime.new(:year<10000>, :month<2>).days-in-month; # OUTPUT: «29␤»