Deactivate your process.
Highlight your Add Reminder action, go in to the Properties pane and click the ... button next to Action Instance Data.
Right click on the Send Date field and select Calculation.
Next to Value =, enter DateTime.UtcNow so that it reads Value = DateTime.UtcNow.
If the Add Reminder is in a status optional actions, highlight the status and go in to the Properties pane, click the ... next to Optional Action Instances.
Highlight Add Reminder and click the ... button next to Actional Instance Data and then add the calculation as above.
Hope that helps,
If the calculation is set as you had described, will the analyst still be able to set the date to something else?
Disregard my suggestion, I think only it works with Automatic Actions, not manual or Optional Action Instances.