Implement fix for timezone and address comments

As for as we can tell, the process for generating realtime timestamps
used to go:

- generate timestamp now for utc e.g. 11:20 am +01:00 when BST would be 10:20 am +00:00
- change timezone to gmt without affecting the rest of the time e.g. 10:20 am +01:00
- send to backdrop
- backdrop converts to UTC but gets confused by the offset and thinks UTC is 09:20 +00:00
- save, timestamps as they are now will be sent to spotlight.

This puts the time coming to spotlight one hour back vs local time

The fix:

- generate timestamp now for utc e.g. 11:20 am +01:00 when BST would be
  10:20 am +00:00
- send to backdrop
- backdrop converts to UTC but this makes no difference
- save, timestamps as they are now will be sent to spotlight.

Tests force the system time to a summer time and winter one and check we
are sending the correct timestamp to backdrop.