You can have a scenario watch new items (like your Google Calendar “Watch Events” module) and schedule a cron job callback using apps like 0CodeKit that allows you to schedule a “callback” to a URL defined in a “Custom Webhook” trigger in another scenario at the exact time you want.
In your second scenario, you can send the SMS when the webhook trigger module is triggered by the schedule.
You don’t have to use a data store, this is just an example. You can replace with Google Sheets, Google Calendar, etc.