A random date in the past is chosen, and geohashers race to reach that date's geohash. The game is played somewhat similarly to "capture the flag", as if the location of the flag was identified at time zero to begin the game.
- Two or more teams get together at a neutral location. We'll call this base camp. This could be near the center of the graticule, but is not necessary.
- A date is determined randomly, in a way that all the teams learn what that date is simultaneously.
- The teams, armed only with the date, must identify the location of the geohash for that date, get there, and verify that they have reached it first, such as by calling base camp, or emailing a photo from the geohash. Specifics would be agreed to before the race begins. At worst, the second team arrives to discover the first team is already there, but in practice there are other ways to prove where you are.
- If the location is unreachable, a new location should be determined, either by picking another date or using the closest-approachable point instead.
The game can be played across multiple graticules, with each team needing to reach the geohash in their graticule. This may not be completely fair unless all teams start at the same place relative to their graticule. (Otherwise it's a coin toss depending on which team starts closest.)
Special limitations can be applied so that all teams are equally equipped and/or handicapped. For example, all teams must have a GPS, or must travel only by bicycle or on foot.
Two geohashers walk into a bar. (If you're still reading, good :-) They walk over to an unoccupied table. One rolls a d12 for the month and a d20 for the month day. The other rolls a d8 for the year, and adds 2000. They start off. If they discover that the location is completely unreachable, (out at sea, in a war zone, etc.), they notify each other, go back to the bar, have another pint and roll again. When one reaches the geohash, he notifies the other and proves it.
The race essentially starts as a road rally. If everyone starts by leaving the same location, it'll literally be an episode of Speed Racer. Perhaps the date could be determined by a riddle, given to all teams simultaneously but requiring some time to solve, giving some advantage to the team solving the riddle first.
Unlike geohashing, where you get to take your time and enjoy nature, play games and clown around, retrohash racing is too much like any other road racing. I don't believe most people would have a problem with something competitive in principal, but this approach to geohashing is not recommended.