In the new Outlook for MS365, you can bypass the limited dropdown menu by manually typing your desired time directly into the reminder field. Open your calendar event, click the reminder dropdown, and instead of selecting a preset, click inside the box and type a value like "2 weeks" or "1 month" then press Enter. This manual override is the quickest way to set a duration that exceeds the standard one-week limit.
If the manual typing method does not save correctly in your specific version, you should use the email reminder feature located under the event's more options menu. By selecting the add email reminder button and choosing the custom option, you can enter a specific number of days or weeks for a notification that arrives in your inbox. This serves as a reliable alternative for long-term alerts that the standard pop-up system might restrict.
Another workaround is to create a separate preparation appointment on your calendar for the specific day you want to be notified. By setting a zero-minute or fifteen-minute reminder on this secondary event, you effectively trigger a pop-up alert exactly when you need it, regardless of how far in advance that is from the actual deadline.
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hth
Marcin