Rikk I spent several hours trying to track down where the XMP entry photoshop:DateCreated="2019-04-23T14:07:54" with no time zone offset was made.
I used Lightroom Desktop Save To: and copied one stack to another area of my disk, then deleted these files from both LRCC and LRClassic. I then imported the original raw files directly into LRCC from the XQD card, first via the iMac desktop with a card reader, then via an iPadPro and a card reader and Apple Photos, then via the iMac desktop directly from the Z7 camera. I did not touch the files with LrClassic. I merged in LRCC. In all cases the merged time zone was correct.
In the first case, I then also opened LRClassic, made an exposure curve change, and let Classic synch and closed it. Then re-opened LRCC and merged again. The time zone was correct.
In other words I was unable to reproduce the earlier result with the workflow I believe I used. When I deleted the stack (after saving them outside of LR for reference) and reimported the original saved files, a merge did result in an incorrect time zone for the dng as before.
In the XMP for one of the nef files, as you have noted, the entries are different for the case with an incorrect merged time zone and a correct one, respectively:
photoshop:DateCreated="2019-04-23T14:07:54"
photoshop:DateCreated="2019-04-23T07:07:54.500-07:00"
I am sorry I cannot tell you which app or sequence results in the entry with no time zone offset, but the fact that the tag is “photoshop” and that I am only using Adobe software suggest strongly that it is an Adobe product that (in some circumstances) does not provide a +00:00 or Z indication for the UT (GMT) time zone the camera was using.
The following is from the Wikipedia entry on the ISO 8601 standard which defines how Date/Time information is to be formatted:
“If no UTC relation information is given with a time representation, the time is assumed to be in local time. While it may be safe to assume local time when communicating in the same time zone, it is ambiguous when used in communicating across different time zones. Even within a single geographic time zone, some local times will be ambiguous if the region observes daylight saving time. It is usually preferable to indicate a time zone (zone designator) using the standard's notation.”
and
“An offset of zero, in addition to having the special representation "Z", can also be stated numerically as "+00:00", "+0000", or "+00”.”
The Lightroom Desktop merge operation appears to strictly follow the standard where no offset information means local time. As I noted it would seem likely that some other Adobe operation is incorrectly dropping the offset information for the UT(GMT) time zone. Given the ambiguity that the Wikipedia article notes and that one is dealing with multiple time zones in Lightroom, you might consider having the Lightroom Desktop merge operation recognize the lack of an offset as meaning Z or +00:00 (UT/GMT).
... View more