I worked out these intervals to be:
Interval Months Weeks Type Leapday Jan-Oct 9 39 3Q no Jan-Apr 3 13 Q Yes Feb-Mar 1 4 M n Feb-Aug 6 26 2Q Y Mar-Nov 8 35 3Q-M - Apr-Jul 3 13 Q - May-Jan 8 35 3Q-M - Jun-Feb 8 35 3Q-M - Jul-Sep + 14 61 5Q-M n Jul-Jun 11 48 4Q-M Y Aug-May 9 39 3Q n Aug-Oct + 14 61 5Q-M Y Sep-Dec 3 13 Q - Oct-Apr 6 26 2Q n Oct-Sep 11 48 4Q-M Y Nov-Aug 9 39 3Q n Nov-May 6 26 2Q Y Dec-Jun 6 26 2Q n Dec-Mar 3 13 Q Y An interval marked '+' is more than a year long.
I classified the interval of 1 month = 4 weeks as M and
the interval of 3 months = 13 weeks as Q.
All the other types are made from adding or subtracting these two types
as shown in the type column
(e.g. 3Q-M is
three Qs less an M).
There are 7 types of interval:
M, Q, 2Q, 3Q-M, 3Q, 4Q-M and 5Q-Mof which
Q, 2Q and 5Q-M may or may not contain a leap day. M, 3Q-M and 3Q never contain a leap day and 4Q-M always contains a leap day.
Karl Palmen
Fri 13 December 2002