When an event shows the wrong time or link in someone's calendar app, start with the event in AddCal. If it is right there, the cause is almost always a setting in the calendar app.
Check the event first
Open the event in AddCal and check its Timezone. Each event keeps its own time zone, which defaults to the calendar's, so an event created in the wrong time zone is off for everyone. See How time zones work for events and subscribers.
Check the calendar app's time zone
If the event is right in AddCal, ask the person who sees the wrong time to check:
Google Calendar: open Settings and check Primary time zone. It should match where they are.
Zones labelled CST: Mexico City and Regina are both labelled CST but do not change for daylight saving. In the US, pick Central Time - Chicago instead.
Phones: turn on Use device time zone in the calendar app's settings.
Yahoo Calendar: Yahoo shows every event in the Yahoo account's time zone. Set the account's time zone in Yahoo Calendar settings. Changing the event's time zone in AddCal does not change this.
A difference of a whole number of hours usually means the app or device is set to a different time zone from the one the person is in.
Apple Calendar shows a different link
Apple Calendar uses the web address behind a link in the description, not the text you can see. In plain-text descriptions, AddCal turns links into footnotes that show the real address. Check the address behind the link, update the event description if it is wrong, and ask people to add the event again.
Changes do not appear
Added events are copies. Someone who added a single event to their calendar has a copy from that moment. If you change the time later, they need to delete it and add it again from the link.
Subscribers update on their app's schedule. Apple Calendar usually refreshes within an hour or two, and Google Calendar can take up to 24 hours. These are typical times, not guarantees. See Calendar subscriber update delay.
Subscribers cannot edit times. A subscribed calendar is read-only, so changes must be made by you in AddCal.
Still wrong?
Contact support with a screenshot of the event in the calendar app, the name of the app, and the device it is on.