You are right on this point! mathematics cannot be fooled :), and this is one of the many problems with the traditional calendar. in this case, shycronization, the start of the week does not correspond to the start of the month, it is a lunar calendar which counts the solar. and although the time difference between them is 11 days in reality. both contain 12 months and not 13 months.
the 13 month is added in the Hebrew calendar every 7 years in the lunar of 354 to catch up with the seasons
https://en.wikipedia.org/wiki/Hebrew_calendar
God creates the world in six days (measure), he perhaps rested on the seventh, and did something else on the eighth and so on on the ninth :)
Amen 🙏