That suggests you may have mixed data formats in the source (ie some data is stored as dates, whilst others are stored as text) or, if there's more than one instance of the date in the document, that the field switch has been entered incorrectly for the one producing the wrong output.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

The issue is not mixed data format in the source as I have checked this (I even recreated the column from scratch). Also there isn't any problem with the coding for the switch - my mail merge is to create letters that contain the date ie the date format is correct on some letters but not others.

I don't think I have explained my problem very well. Below is an example of the output of my mail merge:

10/01/2013 Tuesday, 1 October 2013 (This is incorrect)
15/01/2013 Tuesday, 15 January 2013 (This is correct)
05/03/2013 Friday, 3 May 2013 (This is incorrect)
27/03/2013 Wednesday, 27 March 2013 (This is correct)

It seems that the Word can only create the date in the format that I want when the day is greater than "12".

Surely there has to be a work around.