BEFORE takes two dates, from either an information or in ISO8601 format, and checks if the first one comes before the second. If it does, true will be returned. Otherwise false will be returned.
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Example
e is Entity
BEFORE(e.date_of_founding, '2016-02-29')
This expression checks whether a company was founded before 29th February 2016 (the actual founding date of Atfinity AG; so also if it's older than Atfinity). If it was, this expression will return true.