date when feature opened or the construction of the feature finished
start_date=* can be used to indicate the date the feature opened or construction of the feature finished (i.e. started to exist as feature). See end_date=* for the date when availability of the feature ended. These tags are useful when considering Historical mapping.
Dates use the ISO 8601 system, which is based on the Gregorian calendar. Be aware that for exact dates there are differences of up to 11 days between the Gregorian and Julian calendars and that different countries switched to the Gregorian calendar over a 400 year period.
- 2010 ; where the year is known, but no more
- 0897 (for 897 AD)
- 1848-07 ; where the year and month are known
- 2010-03-31 ; for where the full date is known, using the format yyyy-mm-dd
Caution: ISO8601 uses the format yyyy-mm-dd, not the common North American model of yyyy-dd-mm
Often a date will not be known exactly. In which case some of the following can be used:
- ~1855 (some time around 1855)
- 1860s (during the 1860s)
- ~1940s (probably during the 1940s)
- 480 BC (for something that happened in that year)
- before 1855 (during 1854 or before)
- before 1910-01-20 (before a specific date - possibly when a photo was taken)
- after 1823 (after 1 January 1823 - possibly based on the year in which a map was produced)
- C18 (during the 18th century
- mid C14 (some time during the middle of the 18th century
- late 1920s
- ~C13 (probably in the 13th century)
- 1914..1918 indicates some time during WW1.
- 2008-08-08..2008-08-24 indicates some time during the Beijing Olympics.
- mid C17..late C17
- j:1918-01-31 (a date using the Julian calendar, equivalent to 1918-02-13 in the Gregorian calendar)
- jd:2455511 (Using the wikipedia:Julian Day system, equivalent to 2010-11-10 in the Gregorian calendar)
- Muslim calendar?
'1800s' means the years 1800-1809, not 1800-1899. Prefixes of 'early ', 'mid ', 'late ', 'before ' or 'after ' to indicate those things.
Avoid seasons. For example 'summer 1998' as these vary between the hemispheres. Consider using '1998-05..1998-08' for summer in the northern hemisphere.
BCE can be used in place of BC. AD is assumed where BC is not used but optionally be included.
Avoid multiple entries, for example 2010-08-01;2010-08-19 without providing at least an explanation in a note.
Marking a civilization, period and era
You can also add information by setting the following keys (no need to restrict them to features with a historic=*):
- historic:civilization=* please see the documentation which values are suggested
- historic:period=* please see the documentation which values are suggested
- historic:era=* please see the documentation which values are suggested
Some values drawn from a review of Taginfo: start_date, on 2010-11-10, showing severe heterogeneity.
- OSM-4D for ideas about a viewer showing features in 3D extended over time.
- WikiProject Historic Britain
- Mapping History on Open Street Map presentation (SOTM 2009)