How do timestamps on tickets work with Daylight Savings Time?
Zendesk stores all of its data in UTC time. Everything, including the API, and any emails or ticket events, has a hardcoded UTC timestamp attached to it.
The agent UI is designed to convert the hardcoded UTC timestamp to the user's current time zone. As such, tickets created prior to or after DST may appear to be off by an hour depending on when you look back on them.
For example, an agent in Pacific Time makes an update at 10 am on Friday. Daylight Savings Time changes over the weekend, and they check their update again on Monday. On Monday, the update will appear to be at 11 am. The update actually occurred at 18:00 UTC. On Friday, the agent was at UTC-8, but on Monday, they're at UTC-7. The underlying data did not change. Zendesk's display did.
This is expected behavior and the issue only exists within the agent UI. It is not present in the API which uses the hardcoded UTC timestamp or reporting which maintains the correct historical timestamp.