Use a formula field with DATETIME_DIFF that calculates the difference between NOW() and your reminder time.
Keep in mind that NOW() is not updated continuously. It can take a few minutes to update if the base is open. It can take an hour or longer if the base is closed. This also means that NOW() is almost always in the past, and you can get negative values when you use it with DATETIME_DIFF, and you will likely never get exactly zero.
So if NOW is not reliable for my purpose, does anyone have a way to trigger an automation based on the time in a field?
My use case is I set up an appointment for a certain date & time, I have a duration field that I enter how many minutes before the appointment I want to be reminded, I want to trigger Twilio to send me a text at the reminder time. It really can’t be dependent on whether AirTable is open and can maybe be off by a minute or so but not 5-10 minutes.