I had been doing some recalculation of my calendar, when it occured to me that it would be much simpler to simply have month roughly match up to lunar month. Synodic month is 29,5 days. But with 7-day week, one moth could be four weeks or 28 days. Now, with 29,5 day month, one year would have 12,37 months; with 28 day month, it would have almost exactly 13 months. But would that cause problems - there must be a reason why 12-month year is standard, possibly due to advantage of easy division into quarters? Also, I have already achieved a perennial calendar with a 12-month year, albeit through usage of intercalary days.