The math seems fine to me.

Database Time of:
10/20 1:28 AM -- UTC TIME

Minus:
6 hours -- America/Regina, aka, Capitol of Saskatchewan, Canada

Equals:
10/19 7:28 PM -- Time displayed to guests

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By using relative time stamps, you are choosing to display times as hours/minutes RELATIVE to whatever the user's current time is. So, if you posted something an hour ago, it will display in relative time as, "1 hour ago" regardless of whatever you've set timezone to.

But if you disabled relative time stamps, each user would see the posted time (UTC) translated to match their current time offset. Guests would use the time offset you've preselected for them. In this case, you've selected "UTC-06:00 - America/Regina" for your guests.