Here are the date format id's within Emercury to be used with your List ID or Audience ID. 

This is the format of your optin date. This is important.

You will only need to enter the ID number and not actually use the word ID. (i.e. if the ID is 1. then you enter 1. 

Format id: 1 is mm/dd/yyyy
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1 is "mm/dd/yyyy" 

2 is "mm-dd-yyyy"

3 is "mm.dd.yyyy"

4 is "mm/dd/yy"

5 is "mm-dd-yy"

6 is "mm.dd.yy"

7 is "dd/mm/yyyy"

8 is "dd-mm-yyyy"

9 is "dd.mm.yyyy"

10 is "dd/mm/yy"

11 is "dd-mm-yy"

12 is "dd.mm.yy"

13 is "yyyy/mm/dd"

14 is "yyyy-mm-dd"

15 is "yyyy.mm.dd"

16 is "yy/mm/dd"

17 is "yy-mm-dd"

18 is "yy.mm.dd"

19 is "mon dd yyyy (e.g. sep 1 2012)"

20 is "mon-dd-yyyy (e.g. apr-12-2012)"

21 is "month dd yyyy (e.g. September 1 2012)"

22 is "month-dd-yyyy (e.g. April-12-2012)"

23 is "dd mon yyyy (e.g. 1 sep 2012)"

24 is "dd-mon-yyyy (e.g. 12-apr-2012)"

25 is "dd month yyyy (e.g. 1 September 2012)"

26 is "dd-month-yyyy (e.g. 12-April-2012)"

28 is "mm/dd/yyyy hh:mm:ss"