see
here[
^] for a list of formats that you can use.
here are some options:
DateTime.UtcNow.ToString("r")
DateTime.UtcNow.ToString("ddd, dd'-'MMM'-'yyyy HH':'mm':'ss 'GMT'")
you definitely want UtcNow though because that will convert it to coordinated universal time which "in casual use, when fractions of a second are not important, Greenwich Mean Time (GMT) can be considered equivalent to UTC or UT1" from
http://en.wikipedia.org/wiki/Coordinated_Universal_Time[
^]