The achievement can be claimed if you venture to two or more geohashing locations on the same day and the same local time. If you don't own a time-machine, you'll probably need to cross a time-zone border. Of course, you need to upload photos of yourself containing the necessary proof.
|This user earned the Time-traveller achievement|
- Woodveil was at the 2009-03-08 33 -84 and 2009-03-08 33 -85 hashes at 12:39 local time.
- B2c and Patrizius were at the 2013-10-27 47 13 and 2013-10-27 47 12 hashes at 02:09 local time. It was a DST time travel.
 See also
Multihash -- reach multiple geohashes on one day