A month is NOT always 4 weeks. 4 weeks * 7 days = 28 days. What about:
Jan (31 days) - missing 3
Feb (the only month to match this - except once every 4 years)
March (31 days) - missing 3
April (30 days) - missing 2
May (31 days) - missing 3
June (30 days) - missing 2
July (31 days) - missing 3
August (31 days) - missing 3
September (30 days) - missing 2
October (31 days) - missing 3
November (30 days) - missing 2
December (31 days) - missing 3
Total of missing days:
3 + 3 + 2 + 3 + 2 + 3 + 3 + 2 + 3 + 2 + 3 = 29
If you can get the world to use a 30 day month in its calendar then and only then will you be able to use a real and true uniform system for this and yes, I AM aware that SOME calendars in use DO go by the 30 day/month system.
