You must write dates correctly.
I had students who thought it amusing to write dates like this:
Monday, the 11th day of the month of September, in the Year of our Lord, AD 2001
Another student would write: Feast Day of St William of Norwich, ADMMVII (26 March 2007).
The way to write a date is 11 September 2001.
This way you eliminate all commas and all abbreviations like st, nd, rd and th as in 1st, 2nd, 3rd and 4th. Good writing is about editing-out all the distractions to the eye and making every number, word, phrase and sentence as easy and quick to read as possible. HOWEVER, always study the publication you are writing for and check its style for dates. Some publications prefer September 11, 2001.
But you won’t find a mainstream professional publication that publishes dates like 7.2.06 or 07-02-06 or 7/2/06 or dates with st, nd, rd and th. You should NOT write in these styles, even in an assignment or in a letter. Get into the HABIT of doing it the journalistic way. Stick to mainstream journalism styles, even on a letter.
Incidentally a date like 9/11/01 is very confusing. In Australia and England 9/11 means 9 November (the 9th day of the 11th month), but in America 9/11 it means September 11 (the 9th month and the 11th day). So, please for clarity and ease of reading, write it plainly, full out, even if writing a simple note or a personal email.