@Jeff_Hager, here is my solution for this issue although I think, both solutions complement each other.
In my case, I have a Google Sheet with a timezone XX and I try to have on APP the time based on that GS timezone and not the user timezone which is used by Glide.
If my GS is using “Europe/Berlin” timezone (GMT +01) and my APP has multizone users around the world (USA, Brazil, Spain or Venezuela) buying tickets or making a booking I’m going to need to have transactions timestamp based on my GS (Berlin) always to have a real order and know the sales queue correctly (based on time).
Otherwise, If I use the Glide time (based on device/user timezone) my transaction log will be hard to understand and sort out.
The key in my case is write and use the GS timezone into a cell using the Now() function. This will rule everything regarding time and date. It’s the only thing you will need to use in your GS.
… . 
Later, in GDE you will have these 3 new columns with a simple logic/math:
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Glide Time (Local Time)
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Time Offset
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new Glide Time (based on GS Timezone)
And trying to replicate your APP demo 
, you can see what happens on my APP verifying several timezones and times:
https://glide-time.glideapp.io
In my case, I’m in Venezuela and have -6 hr with Berlin and @Mark_Turrell’s house 
.
With it, any transaction done by me (or any user) from APP will be logged using Berlin’s time instead of local time (user timezone).
Although few people and in few times the GS’ timezone will be changed frequently, I created a choice list to carry out it automatically and test better my solution (using a small script as well). None of you needs to have it on your APP to get this functionality, this is not mandatory.
Again, I created the timezone choice list and my script to make this APP demo more friendly only.
I hope it helps anyone.
Saludos!.