When the 'Birth date' merge field is used in a constituent merge letter, it doesn't merge in the correct format. This is even the case when using a date format switch in Word.
We are currently evaluating this issue for a fix in a future service pack.
Steps to Duplicate
IE 11, Word 2016
Steps to duplicate
1. Create a constituent export defintion that includes the 'Birth date' field 2. Generate a header file from the above export definition 3. Usng your header file, create a merge letter in Word, that includes the 'Birth date' merge field, add the merge letter to a letter template 4. From a constituent record in CRM, click 'Write a letter' and use the letter you just created 5. From the Documentation tab, click the link to create the merge letter
Notice the birth date merge field displays in the format yyyyMMdd e.g. 19800322 (22nd March 1980) when it should be 22/03/1980