After a bit of work, I finally got DateTime to recognize the time value from Time::HiRes and format it correctly for my test database.
use aliased 'DateTime::Format::Strptime';
my $format = Strptime->new( pattern => "%FT%H:%M:%S.%5N" );
# this is the format that Time::HiRes will return the time()
my $datetime = DateTime->from_epoch( epoch => 1221753628.79859 );
You can actually get finer granularity from Time::HiRes, but my god, this is for the start and end of each test program. You won't need finer granularity than this.