10 September 2011
you can convert it online too. http://www.google.co.in/search?rlz=1C1CHNY_en-ININ424IN424&aq=1&oq=excel+to+xml+converter+online&sourceid=chrome&ie=UTF-8&q=excel+to+xml+converter+online+free
if you are doing it for income tax return filing than use ITR utility which is available on income tax web site.
The due date for submission of ITR-V for A.Y. 2010-11 has been extended upto 31.10.2011 or 120 days from the date of upload whichever is later.