We don't have a specific Time field you can use right now, but do have one on the product roadmap to be added soon.
There may be other ways to accomplish this. One method would be to leverage the power of ONTRAPORT's Tasks to track the appointments.
If you create a Task for each appointment, and have you or a member of your team complete the Task when the client arrives for their appointment - this can be done using a form outside of ONTRAPORT - then your follow up email can be part of the "Appointment Campaign".
In our article on Tasks at this link, we include this image that shows how a Task being completed can have a Goal or Goals right after it based on the "Task Outcome". In this image the Goals are for the "Partner Application" being approved or not approved - "approved" is one possible Task Outcome and "not approved" is another. When the team member completes the Task the appropriate Goal is achieved and the automation below that Goal is started.
If you set a Task of "Showed Up for Appointment" with a "Yes" or "No" outcome, the "Task is Completed - Yes" Goal could lead to a Wait for one hour and then send the follow up email. The "Task is Completed - No Show" Goal would likewise allow you to send an email immediately that asks the client why they didn't show up, create a new Task for someone to bill the person for the no show, etc.