This page is intended to provide facts and rules about retro expeditions. It is compiled from various sources on the wiki as well as discussions on talk pages and the IRC channel.
 About Retro Expeditions
 What's a retro hash?
Visiting coordinates of another day than the one you actually go there is called retro hashing.
In a number of cases - e.g. on the weekend - it may even be possible to visit future coordinates. Although the term retro only suggests past dates, it also includes such an inverse retro expedition.
 When, why and where can I go retro hashing?
Go whenever you are really bored. Choose wisely on a point.
Theoretically you could choose every point from history as a destination. But neither it is very sporting nor interesting to just choose some convenient location, so whenever you have a choice, you really should go for the coordinates of the actual day. Taking your chances on just the one day where the coordinates apply is a vital part of the sports of geohashing.
However, a number of special dates in the past may be worth to celebrate by visiting their coordinates. These could include one's birthday, wedding day and the day from the original comic, or other memorable dates in the past.
Also, if you plan an actual meetup on a specific day and the coordinates just are impossible to reach, it may be a good idea to keep it in the spirit of geohashing by choosing the alternate coordinates from a set of retro coordinates (e.g. from the week before) instead of just meeting at a convenient but arbitrary place.
In each case, don't forget to tell us why you chose just that day.
 Coordinates for retro expeditions
The peeron map and many other tools listed on the Implementations page allow to enter old dates. Please consider the rules about 30W-adjustment below. Also, some tools which claim to be fully 30W-compliant are not if it comes to retro hashes. If in doubt, use the peeron map.
30W Time Zone Rule for retro hashes:
The 30W Time Zone Rule is only in effect since 2008-05-27.
- Coordinates for dates from introduction of the DJIA until 2008-05-26 are calculated using the actual day's DJIA. 30W adjustment is not used, all graticules use the same fractions.
- For dates starting from 2008-05-27 and graticules east of the 30°W line the 30W adjusted coordinates are valid as they were on the actual day.
30W Time Zone Rule for retro Globalhashes:
The rules for the globalhash state that all globalhashes are calculated with 30W adjustment, so this also applies for retro globalhashes. This is also how the a globalhash lookup map would it. (But not DrWilco which is badly broken and gets the DJIA completely wrong.)
 Reporting Retro Expeditions
Let us know about your adventures!
But always keep in mind that a retro expedition never is a regular expedition, which is also expressed by a few rules on naming and categorising. Generally, for a successful geohash expedition you need to go on the right day.
- Name the page after the date of the coordinates (and not the date on which you actually went on the expedition). So if you went to the exact spot of the original comic, the page would be named 2005-05-26 37 -122 no matter when you actually went there. If the page already exists, add your report, even if that means to combine regular and retro expeditions into one page.
- In the text (e.g. the section title) mark your expedition clearly as a retro expedition and add the date when you actually went there.
- In the Meetup graticule template set the flag retro = 1.
- For the expedition page, use only the following categories:
- Category:Retro meetup (is set by template)
- Category:Meetup in LAT LON (is set by template)
- Category:Retro coordinates reached if you reached the retro coordinates.
- One of the other Failed categories if you didn't reach the coordinates.
- Category:Expeditions with photos if applicable
- Category:Expeditions with videos if applicable
- For pictures, use:
- Category:Retro meetup
- Category:Meetup in LAT LON
Do not categorize your page or pictures into any of Category:Meetup on yyyy-mm-dd or into Category: Expeditions. These categories are automatically evaluated and are reserved for regular expeditions.
 Achievements for Retro Expeditions
Generally no regular achievements can be earned with a retro expedition. This also applies to achievements which don't require you to reach the coordinates, and also to the Virgin graticule achievement. A graticule where there have been retro expeditions but no successful regular expeditions by now is still considered virgin.
However, there is a small number of special achievements that can only be earned on a retro expedition. These usually can't be combined with any other achievement.
- Origin geohash achievement - visit the coordinates of your day of birth
- Wedding Anniversary Geohash - visit the coordinates of your wedding day
- On Geohashing Day you may celebrate at the coordinates of an earlier geohashing day if the current year's coordinates are no good.
For achievements which allow going to (voted) alternate coordinates under special circumstances, namely the XKCD Centurion, these alternate coordinates may of course be retro coordinates. So it might be possible e.g. to combine XKCD Centurion and Origin geohash achievement by convincing others to meet you on your origin coordinates on a Saturday afternoon.