Add description of three new DateTime methods

doc/Type/DateTime.pod6

@@ -252,6 +252,52 @@ Returns an L<Instant|/type/Instant> object based on the invocant.
 
     say DateTime.new('2015-12-24T12:23:00+0200').Instant; # OUTPUT: «Instant:1450952616␤»
 
+=head2 method day-fraction
+
+Defined as:
+
+    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 C<day-fraction> value is the same as the fractional part of
+the C<modified-julian-date> for the same instant.
+
+=head2 method julian-date
+
+Defined as:
+
+    method julian-date(DateTime:D: --> Real:D)
+
+Returns the L<Julian date|https://en.wikipedia.org/wiki/Julian_day> (JD) for the UTC date and time.
+
+    say DateTime.new('2021-12-24T12:23:00.43Z').julian-date; # OUTPUT: «2459573.0159772␤»
+
+The C<julian-date> starts at zero at the epoch of noon UTC on
+I<Monday, January 1, 4713 BC> (-4713-01-01T12:00:00Z), on the Gregorian calendar (the calendar
+in use in much of the world and in international commerce and travel)
+and is used in astronomy to define times of celestial objects transiting the
+Earth's Prime Meridian. For an instant, it is the sum of the number of whole days and
+the fraction of a day from that epoch to that instant.
+
+=head2 method modified-julian-date
+
+Defined as:
+
+    method modified-julian-date(DateTime:D: --> Real:D)
+
+Returns the L<Modified Julian Date|https://en.wikipedia.org/wiki/Julian_day> (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 C<modified-julian-date> is same value as the C<day-fraction> for the same instant.
+Likewise, the integral part of the I<MJD> is the same value as the C<daycount> for the same instant since they
+reference the same epoch (Nov. 17, 1858).
+The MJD is obtained by subtracting the constant C<2_400_000.5> from the I<Julian Date> and is used to simplify
+transformations between civil and astronmical time systems.
+
 =head2 method posix
 
 Defined as: