Why do calendar reminders arrive at an unexpected time?

Begin with the event itself, not the notification settings. Open the event and confirm its date, start time, and reminder lead time—for example, an alert set for 30 minutes before an event will arrive earlier than one set for 10 minutes before. In Google Calendar on a computer, an event can have its own notification setting, so it may not match the calendar’s default. Check whether the alert is a notification or an email, too. [S1]

Write down the event time you expected and the alert time you actually saw. That can help you distinguish an unexpectedly early reminder from an event whose displayed time differs from what you expected.

Compare the time shown for the event with the calendar’s displayed time zone. Google Calendar’s general settings include a main calendar time zone. On iPhone and iPad, event times are shown in local time, and a device-time-zone option can update the calendar’s time zone while traveling. [S2] [S3]

For example, if you arranged an event for 9:00 a.m. in one time zone but are now viewing the calendar while traveling, check the displayed time and the calendar’s time-zone setting before changing the reminder. Don’t assume the reminder is wrong solely because its clock time differs from what you expected in another location. These settings are useful checks, but they do not establish the cause of every surprising alert.

Check notification settings at more than one level. In Google Calendar, calendar-level defaults apply to new events unless an individual event has a custom setting. Account-level controls can affect notification delivery and snooze timing. On a computer, browser permissions and whether Calendar remains open can also matter for desktop alerts. [S1]

Inspect the event’s reminder, then the calendar’s event and all-day-event defaults, followed by account and browser notification controls. If the alert was snoozed, note that separately, since snooze timing is one of the documented account-level controls. Avoid changing several settings at once: make one change, then observe the next relevant reminder.

Use a brief note like this, filling in only what you can verify:

“Calendar/app: [name]. Device: [computer or phone]. Event shown as: [date, time, and time zone]. Reminder setting: [lead time and notification type]. Alert received: [date and time]. Snoozed: [yes/no/not sure]. Settings checked: [event, calendar defaults, time zone, account, browser or device].”

Then use this order: 1) event date, time, and reminder; 2) calendar time zone; 3) calendar defaults; 4) account and browser or device notification controls; 5) any snooze action. The documented Google Calendar settings identify relevant checks, but do not establish the cause of a particular alert or cover every calendar app and device. For another product, look for equivalent event, time-zone, and notification controls rather than assuming its menus match.