What is Process Execution Time?

I have a Workflow that has this logic:

If Process-Execution Time is > Begin Date (Date Only), then Stop Workflow and display error:  Your Begin Date cannot be before Today.

My question is, what value is in Process-Execution Time; does it include the Date and Time?  And does it compare to the Begin Date? 

This workflow keeps failing if the Begin Date = Today, so I'm thinking I'm not understanding the Process Execution TIme.

My need is to ensure the user hasn't accidentally selected a date before today.

Thanks!