Use the Tools - Macro - "Record New Macro" function in Word, create one called "ProperDate" or whatever you fancy, and do some simple function that you don't need (File, Open, some document would do). Then close the Macro creator (click on the square in the little tool-thing).
Go into Tools - Macro - Macros and find the one you just created. Open it for editing. It will have the "top and tail" that you need. Replace the line that says something like
Documents.Open FileName:="myletter.doc",
- it may be a lot longer than that. Remove everything from that line (probably the next line will be "End Sub".
Replace that line with
Selection.InsertDateTime DateTimeFormat:="d MMMM, yyyy", InsertAsField:= _
False
Job done.
Test it to see it inserts the date.