This is probally time conflict issue on console/PC.
The mission countdown is base on the time and date of your console/PC when you start the mission.
Example: You started it at 12/12/2014 8am GMT. (This is the initial time of your console/PC & actual date & time)
Here some possible outcome. (For a 2 hour mission)
1. You set your console/PC time to 12/12/2014 9am. (Actual time: 12/12/14 8.01am)
result: the mission countdown speed up 1 hour even it just 1 minute passing
2. You set your console/PC time to 11/12/2014 8am
Result: Your mission add on additional 24 hour to the mission and become 26hour countdown
To solve the problem.
Just reset the time to 1 year ahead and get the mission done. After that, don't initiate any mission and save.
Then reset back the time only start any other mission.(To prevent automatic update on time or you will get the mission countdown as 1 year)
