Reference to a cell in an unopened file

I want to put a reference in a workbook to a date in another file that is NOT open at the moment. When I open both files and point to the other file to get to proper file path, file name and cell reference it returns the proper date. However, when I close the referenced file, the formula returns #REF!. The referenced file is ='C:\Users\Public\Documents\Prospect Pricing Tables.xlsx'!Last_Update.

I even tried to refresh the cell in my workbook with this code yet it errors out on the file address line.

'Enter Last_Update date from Master Table as it Opens
Sheets("Output").Select
Range("Master_Date").Select
ActiveCell.Formula = "='C:\Users\Public\Documents\Prospect Pricing Tables.xlsx'!Last_Update"
Range("A1").Select

I would appreciate anyone's help.

Thanks,
RR