Date test VBA - Very easy

Hi this is a really easy one but I don't know how.

Basically I want to add vba on workbook open that will do a date test and show a msg box. I know the workbook_open part and msg box and if this were a formula I would be fine.

Test:

if todays date is greater than **cell a1** then
msgbox "XXXX"

thanks,
dan.