Hi there, @chiara1… this post is probably the best way to go, but you can hack something together if you don’t want to use a plugin. I did something quite a while ago where I got the week number of the second date in the range and subtracted the week number of the first date in the range, multiplied the result by 2, and subtracted that number from the number of days in the range. See, hacky, but it did the trick for my use case.
Hope this helps.
Best…
Mike