Four seven-day weeks were within a day and a fraction of the lunar
month, which was the nearest that could be got. The first method of
measuring the year would be by twelve lunar months, which would bring
it back nearly to the same period. But as the lunar month is 29 days
13 hours, twelve months would be 354 days 12 hours, or nearly eleven
days less than the tropical solar year. Hence if the lunar year was
retained the months would move back round the year by about eleven days
annually. This is what actually happens in the Muhammadan calendar
where the twelve lunar months have been retained and the Muharram
and other festivals come earlier every year by about eleven days.