See https://www.rfc-editor.org/rfc/rfc3339#page-5 section 4.3

Not sure now of the point of using -00:00...

Say we have some country A which is UTC -04:00 i.e. 14:00 in UTC is 10:00 in country A.

And we're making a KDM for country A. We want to enter the validity time there, not here. DoM can't know what time zone a particular country is in, so we must tell it the time zone.

This is what we tried to do so far with the UTC offset in Cinema. But that doesn't work because UTC offset changes through the year.

Now we're suggesting in the UI, we want the KDM to start at 17:00 local time, so we say

17:00 UTC-4

(i.e. 21:00 UTC) and into the KDM we we write 17:00T-04:00.

Not sure what the benefit is of writing 21:00T-00:00 as this report suggests.

This also feels like quite a big change that should maybe end up in 2.17.x

Running tests of 2300 libdcp branch against DoM, then merge that to libdcp main, then merge this to 2.17.x

a7ba9ea0f6a896821f9c9540fd7bbedfc059cf42 in v2.17.x

Doubts were raised about this: https://dcpomatic.com/forum/viewtopic.php?t=2359

If we have TZ per cinema, and we make KDMs, they will be wrong during DST, and there's not much you can do other than edit the cinema.

If we remove TZ per cinema, and set it during KDM creation, you have to remember the TZ + DST otherwise the KDM timing is more wrong.

So the argument is that we should have TZ per cinema, and also allow it to be edited at creation time - if you forget (or don't know) that the cinema is in DST at least you're only an hour or two out.

The difficult part with that is that we can have multiple cinemas (in different time zones) selected, but only one set of controls for the timing window. I guess in those cases we can disable the time zone modifier.