Once you saved the start of the meeting time into custom field, you can use date/time based triggers to set up the reminders before the meeting.

In every reminder, always give the user options to opt-out reminders, and add the tag to the user, and add condition check in every reminder automation. 

Also you can send SMS & Emails reminders if the user is out of 24 hour window. 

If you request one time notification after the appointment is confirmed, you can also use OTN to send reminders if the user is out of 24 hour engagement window.

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