Hi Sander,
I understand.
Please create also a global workflow variable (GV) e.g. called "ReminderDate" <DateTime> and fill it with the reminder DateTime value.
Place a "time delay activity" (or "time interval" - have just a german WF designer frontend and I'm not 100 % sure about the correct designation/description of this activity). Set the radio button to "time delay" and choose your GV "ReminderDate".
Place an "e-mail activity" behind the time delay activity and configure it.
If the current DateTime has reached your reminder DateTime, the workflow will automatically sent out this e-mail.
Best regards,
Frank