I've been working on this problem on an off for a while now, but from the other side as it were. Trying to extract the dates that the record author meant from what they actually wrote.
There are a LOT of different date formats out there and I've yet to see a really good solution.
I'm coming at this problem from a software angle, trying to decode dates automatically, so my desires for date formats may be different to yours.
But I really, really, really wish that date information was stated explicitly and consistently.
Plenty of collections use modifiers like 'circa', 'early' or 'first half', but then don't use these consistently.
In one record 'late 20th century' means 1950 to 2000, in another place it will mean 1975 to 2000.
These sort of date modifiers never seem to get explicitly defined for the collection which means that what one collection means by 'circa' is different to what another collection means.
The modifiers also mean different things to different dates, 'circa 1950' may mean 1945 to 1955 but is `circa 1950s' 1950 to 1959 or 1945 to 1965?
There are lots of records with dates like '80s' where you just have to assume the century information or '1940-50s' where you assume it means 1940 to 1959.
So for me, the best way is just to provide the upper and lower bounds for date period in full, i.e. not `circa 1955' but instead `1950/1/1 to 1959/12/31`.
Or if that's not possible, define exactly what you mean by 'circa', 'late', 'early' etc and make that information available where anyone looking at your records can see it.
For example, are you going to use the word 'circa'? or just put a 'c' on the front of the date i.e. 'c1950'?
If there are two dates in a field does the circa apply to just the first one or both? i.e. is 'circa 1950 to 1960' the same as 'circa 1950 to circa 1960'?
If you are saying 'circa 19th century' do you mean up to 25 years either side? 50 years? 75?
Software can decode any format you use as long as we know what the rules are.
P.S
There are some truly interesting date fields out there and I've been keeping a list as part of my really tricky testing data.
Some of my favourites are '25 feb ?', 'circa pre world war two', 'early or late 19th or 20 century' and 'c18-1 to c--01?'
David
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