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Add option to display time in alternative calendars #890

Open 1ec5 opened 1 month ago

1ec5 commented 1 month ago

Depending on the time period of interest, the proleptic Gregorian calendar isn’t particularly suitable because it requires counting backwards before 1 CE. Additionally, if one is comparing OHM to sources that use, say, the Julian calendar, the time slider becomes more difficult to comprehend.

We should add a new option to switch to an alternative calendar for display purposes. Ideally, the option would live in My Preferences as part of ohm-website. It should affect not only the time slider and inspector but also other static parts of the site – maybe even changeset and element timestamps, to avoid a confusing mix of calendar systems on the site.

In addition to the Julian calendar, this forum thread suggests the Holocene calendar as one that might be useful as a frame of reference in the UI. We could also support calendars with less straightforward conversions, as long as the conversions are precise and formulaic.

This issue is only about frontend display, not backend editing or storage, since the ISO 8601 standard doesn’t provide for alternative calendars at all. Automatically switching between Julian and Gregorian based on the year should probably be out of scope too, because cutover dates were wildly inconsistent around the world and were nuanced or disputed in some cases.