ToUnixTime* methods on DateTimeOffset can be translated in SQL Server as DATEDIFF(second|millisecond, '1970-01-01T00:00:00.000', [Date]).

This can even be done today verbosely with EF.Functions as EF.Functions.DateDiffSecond(DateTimeOffset.UnixEpoch, date). It would be nice if translation of ToUnixTimeSeconds/ToUnixTimeMilliseconds was supported natively.