If you'd like to track unpaid time off, you can do so by going to Settings --> Time Off:
On the Time Off page, you can either add a new time off earning code if needed or Edit an existing code:
This is where you can designate what type of time off code you're creating. In this case you'll want to select Unpaid:
Unpaid time off will show up under a new column on an employees time card:
Just as with paid time off, unpaid time off is not included in overtime calculations.