What timestamp format is this Sqlite column using?

It turns out that the Notes app on an iPhone stores the notes in a sqlite database, and after a bunch of hacking, I was able to get this file onto my PC for backup and analysis. But I'm puzzled because the timestamp values are in this format, which I don't recognize:

-978184314.566524
394221085.203589
412276958.735525
412279753.951386

The first one could be some factory installed default note with a bogus timestamp. All the rest would have been made some time in the iPhone era, after 2010 say.

What format is this, and how to convert it to a human readable date and time?