Ok thanks for sharing the information in detail .first of all please check the if you have any subscription enabled in course product or not in course pricing settings : If there's a subscription set in the course then it will overwrite the course duration :
http://prntscr.com/f1h5ib
Second thing to be noted here that the time for the course starts as soon as user purchases and subscribes to course . So the time when user waits for the unit in which start date is set will also be taken into consideration .
So I guess the user's time may be getting consumed when she's waiting for the first unit on specific date .
As quick fix please extend their subscription from course admin :
http://prntscr.com/f1h7t0