So you put in for PTO, it was approved, and then...   something comes up and you have to come in and work!!

While that certainly puts a damper on that day at the beach, logging your time and backing out  PTO time will not be an issue. 

From a user perspective, you will just need to clock in/out as you normally would.  This can be a partial or full day.   If you are working a full day, you will be auto-clocked out at 7.5 hours as we have set a limit of 15 hours worked for a single day to take into account the possibility of working on a PTO day.   PTO balances will correct, once a manager adjusts/rejects the PTO request. It would also be a good idea to put in the notes field what transpired.  i.e.

"I was approved for PTO 8-12, but came in to work instead"   

or

"I was approved for PTO for the day, but had to come in to finish a grant"

From a manager's perspective, you will need to adjust the users PTO time.  This will either be an edit of the PTO time taken or rejecting the time if none was used.   I have attached a link to the instructions that covers how to do this.