Can someone provide me the VBA for a pop up to display upon the opening of an excel file on or after the date 1/1/2018?

You would use a Workbook_Open event handler such as the following. Copy this code, changing the message as you wish:

Private Sub Workbook_Open()

If Date >= 43101 Then

MsgBox "The day has arrived!"

End If

End Sub

Press ALT + F11 (Function Key F11).

Double click 'This Workbook' in the Microsoft Excel Objects in the upper left quadrant.

Paste the event handler into the white editing area to the right (right click inside the area and 'Paste').

Close the VBE (Red button - top right).

Save the workbook.