This is a combination of two separate but related issues in the Reminders section of the Event Add/Edit screen.
Please refer to this topic: https://groups.io/g/Group_Help/topic/events_calendar_reminders/79560226
The bug part; the "minutes before" duration textbox and/or
underlying code allows the user to add/edit and save a Reminder
with zero or negative values as the "minutes before" duration, no
check or validation is done. Duane tested the actual
notice-generation event part, and it results in notices sent out after
the event has transpired; a 0-min reminder's notice came 5
minutes after the event (the 5 must be significant because that's
the default textbox display value when one adds a new reminder).
These are non-desired results.
The suggestion part; as you'll note in the above linked topic, the current Reminder frame's element layout can foster some ambiguity/fuzziness on adding/editing Reminders for Calendar-inexperienced folk (myself included at first look at it), coupled with thin/insufficient/unclear user-manual Calendar how-to info. From a screen POV, my suggestion would be to rearrange the elements a bit or add some visible separator/delimiter, something to make absolutely clear the checkbox is not related whatsoever to the button above it or the AddReminder process, maybe something like in the attached. Or alternatively maybe leave as is but change the checkbox to say "Send Reminder" instead of "Send Notice"? Not sure which tweak(s) would work the best, but some tweak would help to make things crystal-clear.