Document Date.later/earlier functionality

Author: lizmat

doc/Type/Date.pod

@@ -23,6 +23,7 @@ an object the current day according to the system clock.
     say $d.month;                   # 12
     say $d.day;                     # 24
     say $d.day-of-week;             # 1  (that's Monday)
+    say $d.later(days => 20);       # 2016-01-13
     my $n = Date.new('2015-12-31'); # New Year's Eve
     say $n - $d;                    # 7 days between New Years/Christmas Eve
     say $n + 1;                     # 2016-01-01
@@ -65,6 +66,53 @@ Usage:
 
 Returns a C<Date> object for the current day.
 
+=head2 method later
+
+Defined as:
+
+    method later(Date:D: *%unit)
+
+Usage:
+
+    DATE.later(DELTA)
+
+Returns a Date object based on the current one, but with a date delta
+applied. The date delta can be passed as a named argument where the argument
+name is the unit.
+
+Allowed units are C<day>, C<days>, C<week>, C<weeks>, C<month>, C<months>,
+C<year>, C<years>.
+
+Please note that the special ":2nd" named parameter syntax can be a compact
+and self-documenting way of specifying the delta
+
+    say Date.new('2015-12-24').later(:2years);
+        # 2017-12-24
+
+Since addition of several different time units is not commutative, only one
+unit may be passed.
+
+    my $d = Date.new('2015-02-27');
+    say $d.later(month => 1).later(:2days);  # 2015-03-29
+    say $d.later(days => 2).later(:1month);  # 2015-04-01
+    say $d.later(days => 2).later(:month);   # same, as +True === 1
+
+Negative offsets are allowed, though L<#method earlier> is more idiomatic for
+that.
+
+=head2 method earlier
+
+Defined as:
+
+    method earlier(Date:D: *%unit)
+
+Usage:
+
+    DATE.earlier(DELTA)
+
+Returns a Date object based on the current one, but with a date delta
+towards the past applied. See L<#method later> for usage.
+
 =head2 method truncated-to
 
 Defined as: