Count missing dates

If you have a calendar table with each day in it, you could then link these timesheets to the day of the calendar and count how many records in that month/year do not have a linked timesheet against it for that individual

You can do it with the fields you have already, though it would be more difficult. You can count the records you have for that month, and then have a formula looking at the first of selected month -> first of the next month and counting the days between the 2, then take the two away from each other

let t := this;
let countOfWorkdays := cnt(select timesheets[month(date) = t.month(date)][year(date) = t.year(date)]);
let monthLength := days(date(year(date), month(date), 1), date(year(date), month(date+1), 1));
let countOfOtherDays := monthLength - countOfWorkdays;
countOfOtherDays

Something along these lines should work for you. It will work dynamically through the months, or you can add some fields in to select which month/year you want to see, depends how you want the output to be