Thanks for the tips @kramwe . I managed to get something going that converted the date/time of an input into UNIX (with this plugin), then converted that from a specified timezone into a UTC UNIX with Gaby’s timezoneDB plugin, then converted that UNIX to a readable date by adding the UTC UNIX as seconds to 1st Jan, 1970 at 00:00:00 (as in this post).
It’s a mismatch of things mentioned in all the various threads on timezones.
This final UTC output is actually +1 hour off for reasons I don’t understand, so I subtracted 1 hour from the final UTC time and it works (if someone could explain why I’ll sleep easy tonight). 
Here is the link to the page for other people’s reference.