New Outlook supports preset popup reminders; for an exact custom time, use Add email reminder → Custom, which sends an email rather than a standard popup reminder.
Set a custom email reminder
In new Outlook for Windows:
- Open Calendar.
- Create an event or open one you created.
- Open the Reminder dropdown in the event’s Options section.
- Select Add email reminder.
- Select Custom, enter the required timing, and save the event.
This workaround has important limitations: the reminder is emailed only to you, must be configured manually, and may need to be added separately to each occurrence rather than applying across a recurring series.^1^
Set a standard popup reminder
Arbitrary popup intervals—such as exactly 18 hours, two weeks, or one month—are not currently available in new Outlook’s event reminder list. You can only select one of the displayed presets.^2^
For one event or an entire series:
- Select Calendar and open the event.
- For a recurring event, select View series if the reminder should apply to the series.
- Under Options, open Reminder.
- Select one of the available reminder intervals and save.^3^
To change the default preset for calendar events:
- Go to Settings → Calendar → Events and invitations.
- Under Events you create, open Default reminder.
- Choose an available interval and select Save if prompted.
The default applies to existing meetings and new meetings you create, but an individual event or series can override it.^3^
Outlook for Android
When creating or editing an event:
- Open Calendar and create the event, or open an event you created and select Edit.
- Tap Alert.
- Select one of the alert options shown.
- Save the event.^4^
The available documentation describes choosing from the alert options presented; it does not document entry of an arbitrary interval in Outlook for Android.
For new Outlook for Windows, the available choices are:
- Use a custom email reminder.
- Use the nearest preset popup reminder.
- If installed and available, use classic Outlook, which provides more flexible reminder handling for individual calendar items.^2^
References
- Default event reminders in New Outlook Calendar - Microsoft Q&A
- How do I set a custom reminder time for an event on Outlook Calender? - Microsoft Q&A
- New Outlook (#tab/new-outlook)
- Android (#tab/android)