How to Process DateTime into a Standardized Format When Saved as a Custom Page Type Field?

So there are multiple things for displaying times.

First what do you have in the Setting module:

Settings > System

• Enable time zones
• Server time zone
• Site time zone

Is the fact its saved as different timezone the issue? Or is the time displayed wrong for another user?

Because if you start using dates in different formats it can get quite complex for the user. So the default is that the date times will be displayed in format the User selected in their profile, or if nothing was set what is configured as the Site Time zone.

If you want to display a date time differently you could also use a different control and display using a macro and default format: {% String.FormatString("{0:yyyy-MM-dd HH:mm:ss 'GMT' zzz)}", Value) %}

Which would get you something like: 2020-11-20 20:01:14 GMT +00:00

Well the issue is that the person who designed this system isn't saving the DateTime in UTC. I've been doing some reading and see that storing dates in UTC is standard practice. I was mostly just wondering if there is a way to create a new page type field that will depend on the localized datetime but it will automatically translate the localized into UTC for another page type field. I could remove the other field but it might cause other issues so I'd rather just keep another copy of the date in UTC.

Do you know if it's possible to generate a hidden page type field that would not be user accessible but it would automatically be generated when another field is filled in? Can I do that with a macro or would I need a code behind file to do it?

Are you using the mentioned Time zones feature? If the time zones are set correctly and also the users/editors have the correct time zone set in their account settings, then they should see the time in their time zone.
For the hidden field - yes, you can use depending fields and then use a macro which will read the value from another field.