how to convert date with 'T' to/from string in C#

I used following functions to convert DateTime from/into string:

DATE_OBJ.ToString(DATE_FORMAT);

DateTime.ParseExact(Date_string, DATE_FORMAT, null);

Now I've got to work with follow format 2012-03-20T14:18:25.000+04:00

Which format should I use to convert it correctly to string and generate string like that from DateTime object?