You know, this exact question is why my book has 8-day weeks.
No, seriously. One of the first things I did was fix every month to be exactly four weeks. However, this reduced the length of a year by a fair amount. Enough to where a character's 18th birthday would still leave them as 17 in IRL Earth-years.
I was so paranoid that someone would dive into my book and look for any excuse to silence or invalidate me, that I closed that loophole by having the world use 8-day weeks instead of 7-day weeks, which means my years are now longer than Earth-years, so no. I can shoot that shite down with maths.