Using VBA to Open a File Name that changes slightly

Hello,

I am also trying to do the same thing. But instead of an .xls file it a PowerPoint one. I am using a macro to add a PowerPoint file as an object.
The file's name is also a date. What is the equivalent of WorksheetFunction.Text(TheDate, "YYYY_MM_DD") function in VBA for PowerPoint?

I would appreciate your help.