Can I set up automatic validation for an employee's clock ins/outs?

Plans: Time, Advanced, HR Starter, HR Professional and HR Enterprise. 

Yes, if you activate the clock ins/outs validation,  you also have a setting that you can enable and allows you to set a number of days, after this period, the clock ins/outs of the employee will be validated automatically if the employee has not previously validated them for any reason. Probably because they have forgotten to do so. 

You will find it in the same area where you activate the clock in/out validation. Follow this route and choose the date and time you are interested in: 

Configuration >> Automations >> Validations.