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.
That said, not every account utilizes UTC as their account time zone. The agent UI is designed to convert the hardcoded UTC timestamp to the account's current timezone. 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.
This issue only exists within the agent UI. It is not present in the API which uses the hardcoded UTC timestamp or Insights reports which maintain the correct historical timestamp.
This is not considered a bug, but is an incredibly complex issue that our developers have already investigated and chosen not to include on the roadmap for future updates to the product.