In today's conversion it is: September-XII-MMIX but the Romans themselves in the past would have probably wrote out the number 2009 as MMVIIII
XII/VI/MMIX
The date September 23rd 2009 can also be written as 09-23-2009 and this can be represented by the Roman numerals IX.XXIII.MMIX
Today's conventional conversion of 11th September 2009 into Roman numerals is XI-September-MMIX whether or not the Romans themselves did it this way is questionable.
IX-XIX-MMIX
IX/IV/MMIX
IX-II-MMIX
Today's conventional conversion of 20th September 2009 into Roman numerals is XX-September-MMIX.However, there is strong evidence to support the fact that the Romans themselves would have written the above date as XX-September-MMVIIII.
May=MAI. 12=xij 2009=MMIX therefore: May 12, 2009 in Roman Calendar is written as MAI. xij MMIX
MMIX is the number 2009 in Roman numerals.
The number 2009 in Roman numerals is MMIX
As you asked this question on 15th September 2009, which can also be written as 15-09-2009, this date in Roman numerals would be XV.IX.MMIX
If you write September 09 2009 as 09-09-2009 then you can write using these numerals IX (9) and MM = 2000, so 09-09-2009 becomes IX.IX.MMIX