I've run into the same problem - so far I haven't found a way to modify date formatting at all.
I've also found that a timestamp valid at midnight is returned in an identify request as just the date, with no time information.
Example: October 28, 2013 at 00:00 is returned as just "10/28/2013"
This really should be able to be customized on a per-field/per-service basis, although I would be happy if we could at least modify ArcGIS Server's default formatting.