Here's a context
I have a female friend and we used to have good feeling about each other, but we didn't end up becoming a couple. The problem was ...
Whenever we planed to meet 'at the time' we were supposed to meet, she was always late for the appointment.
Whenever we planed to meet 'at a time' we were supposed to meet, she was always late for the appointment.
Whenever we planed to meet 'at the specific time' we were supposed to meet, she was always late for the appointment.
I'm not sure which one is the most suitable one.