Tenant left on 15th during month to month, should I insist on the full month rent?

[Johann Jells]

Very good tenants left on 15th during month to month after 2 years, leaving the place immaculate. I easily re-rented the vacant unit immediately for the 1st, without having to show it with them in the process of moving out.  Technically a month to month is by the calendar month and legally I can charge them. But should I? A half month vacancy for 26 months is not a bad vacancy rate. Oh, and they're both lawyers!