While I haven't tried them (have work to do) I am not convinced that any of them "work reliably" when you consider changes to and from daylight savings.

In the UK at least daylight savings changes happen at either 1am which becomes 2am or 2am which becomes 1am, so 2am back to midnight can be 1 hour or 3 hours at least once a year.

Unless I am missing something none of these methods convert both midnight and the current time to UTC before doing the difference between them so I would argue that the correct answer is none of them.