An Outlook user asked this question:
Is it possible to select my own reminder time for an event/meeting? Or for a snooze for a reminder? I want to be reminded 20 minutes before the time not 30 or 15!
Sure, you can type in custom reminder times but you need to type it as "20 minutes" (or change the number in the default reminder). You can’t use shortcuts (2d, 2w etc) like you can in other time fields; the reminder needs to be entered with the time period (minutes, hours, days) spelled out: 13 minutes, 11 days, 6 weeks.
You can do this in both Reminders and Snooze, but you can’t use a custom “minutes before start” in Snooze. You’ll need to figure out how many minutes it is to 20 minutes before start and type that in the Snooze field as minutes.