Calculate each year's Rakudo releases

also fill up the dates for 2013.

docs/release_guide.pod

@@ -71,6 +71,13 @@ Dates are based on Parrot's expected release schedule.
   2013-03-21   Rakudo #62
   2013-04-18   Rakudo #63
   2013-05-23   Rakudo #64
+  2013-06-20   Rakudo #65
+  2013-07-18   Rakudo #66
+  2013-08-22   Rakudo #67
+  2013-09-19   Rakudo #68
+  2013-10-17   Rakudo #69
+  2013-11-21   Rakudo #70
+  2013-12-19   Rakudo #71
 
 =head2 Suggested .pm group names for future releases
 

tools/release-dates.pl

@@ -0,0 +1,24 @@
+use v6;
+
+constant release-number-offset = 2010;
+my @first-rn = (
+    25,     # 2010
+    37,     # 2011
+    48,     # 2012
+    60,     # 2013
+);
+
+sub MAIN ($year = Date.today.year) {
+    if $year < 2010 {
+        die "No support for pre-historic release dates";
+    }
+    my $first-rn = @first-rn[release-number-offset - $year]
+                   // 12 * ($year - @first-rn - release-number-offset + 1) + @first-rn[*-1];
+    for 1..12 -> $month {
+        my $d = Date.new($year, $month, 1);
+        ++$d until $d.day-of-week == 2;     # $d is now first Tuesday
+        $d += 14;                           # ... third Tuesday
+        $d += 2;                            # the release is on Thursday
+        say "  $d   Rakudo #", $first-rn + $month - 1;
+    }
+}