For sure. I was thinking more of "normal" applications, as opposed to things like SQL databases. It looks like the SQL standard explicitly calls for the use of the Gregorian calendar for dates, so I think that answers the previous question of "Who else uses the proleptic Gregorian calendar other than programmers of datetime libraries?"
reply"normal applications", no. But there are more platforms besides databases, e.g. programming languages. Any platform that works with dates will have to deal with these weird quirks. And application that works with dates probably would not, although there's always leap seconds...
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