Simple Job Scheduler Class
Just a small comment from me.  I'm not so sure about the need for the method SetJob().  It seems like there is a requirement for this method to be called before the Start() and once called you wouldn't call it again?  

Hence perhaps I might look at moving this to the constructor that would hopefully imply this concept?  Also that way you can't change the job.  This is of course made with the assumption you aren't looking at changing jobs once a new ScheduledJob has been setup.

Also probably what Stylecop might pick up anyway as suggested by Leonid;


I personally wouldn't recommend using an _ for a variable within a method i.e. _nowMin
Same applies for the private method _timer_elapsed()