As the time is constantly changing on the TruckersMP servers, the time remaining for the job will continue going down even whilst your offline. When you first load up TruckersMP and you're in the menu, the time is still the same time as when you left last time you played. Then when you select a job through the menu, as soon as you take the job you will be loaded into the game, you get connected to the TruckersMP server which will advance the time to the servers time. Which will make your job become possibly late. You can either take a small job and sleep or F7 while on that job, to reset the job economy. But all the jobs you take will have to be complete within that session, to avoid to jobs becoming late. Or you can play single player until you're able to afford a truck, then you shouldn't have any issues while doing jobs, if they're complete in the same session of when you started them.