What is the correct format to use for Date/Time in an XML file

I always use the ISO 8601 format (e.g. 2008-10-31T15:07:38.6875000-05:00) -- date.ToString("o"). It is the XSD date format as well. That is the preferred format and a Standard Date and Time Format string, although you can use a manual format string if necessary if you don't want the 'T' between the date and time: date.ToString("yyyy-MM-dd HH:mm:ss");

EDIT: If you are using a generated class from an XSD or Web Service, you can just assign the DateTime instance directly to the class property. If you are writing XML text, then use the above.

EDIT: This is bad advice. Use "o", as above. "s" does the wrong thing.

I always use this:

dateTime.ToUniversalTime().ToString("s");

This is correct if your schema looks like this:

<xs:element name="startdate" type="xs:dateTime"/>

Which would result in:

<startdate>2002-05-30T09:00:00</startdate>

You can get more information here: http://www.w3schools.com/xml/schema_dtypes_date.asp

If you are manually assembling the XML string use var.ToUniversalTime().ToString("yyyy-MM-dd'T'HH:mm:ss.fffffffZ")); That will output the official XML Date Time format. But you don't have to worry about format if you use the built-in serialization methods.