You should have a data type of Shift with a field for start and end (both are date fields). You should not need any data picker for the employee to clock into a particular shift, and should instead just use a search to find the shift (as shifts are created by a manager when making the schedule) based on the start date being the same as current date/time expression (but of course need to make it constrain against month/date/year - for this it is easiest to use a text field representation of the date to constrain against current date/time formatted in same manner). If you expect more than one result, you can create a simple function to have employee click a ‘button’ representing which shift they are clocking in for.

When it comes time to clock out, just find the shift with a filled in ‘clock in time’ field and an empty ‘clock out time’ field.