Importing Unix timestamp into date time field
According to the documentation for the Date field, Feed Me uses date_parse to parse a provided date string into a proper date object. date_parse uses strtotime underneath, which seems to have issues with timestamps.

I just wanted to confirm your timestamp though, as it seems to not produce the correct date for me using:

echo date('Y-m-d H:i:s', 1457685095000);

// Results in
48162-03-15 06:23:20