Looks like the graylog2-server doesn't store the timestamp carried in the GELF message. In fact, there's no timestamp field in the GELFMessage object, even though it's part of the GELF specification.
Speaking of specification, timestamp is specified as being unix timestamp and a decimal. I'm assuming this means it really can also contain decimal fraction, since having the timestamp with a resolution of one second would be a bit coarse...
Moved to: http://jira.graylog2.org/browse/SERVER-39
The GitHub issue tracker is going to be closed. Thanks!