It will be very difficult to extract such statistics from a return separated list of values in a single field--which is what is produced by such a check box set on a single field. A better approach would be to "check in" an employee by creating a related record with the date worked recorded in a date field. That then allows you to count related records to get the total days worked.
It's possible to set up a list of employees with a button to check them in by performing a script that creates such a record. And conditional formatting can make that button look and function just like your check box.