Weird data in the "Released" field

In the Discogs database each release has a Released field that can contain either a year, a year plus month, or a year, month and day or nothing (if the date is unknown). At some point in time Discogs started to enforce the format for this field, but before then it was more or less a free text field. I looked at how often the Released field contained an invalid year: 1766 times, which is surprisingly low.

Some results that I found:

different date notations (DD-MM-YYYY, MM-DD-YYYY) with different separators (-, /, .)
the word "unknown", in various forms, including misspellings
a question mark
whitespace
multiple years
month names
combinations of the above

It is interesting to see that there is a very clear point when invalid dates are no longer seen in the data (likely somewhere in 2016), as this is when the checks were put in place, showing that automated gatekeepers to keep data clean do help.

Checks that I made will soon be added to the cleanup scripts on GitHub.

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