Timer jobs are tasks scheduledto perform specific maintenance tasks in a specified regular time interval.It contains a definition of the service to run and specifies how frequently the service should be started. The Windows SharePoint Services Timer service (SPTimer) runs timer jobs. Many features in SharePoint Products and Technologies rely on timer jobs to run services according to a schedule.
However, there is a problem with the timer job while upgrading the solution.If due to some modification or latest changes the user wants to upgrade the existing timer job solution,then the timer job will not recognize the latest changes and it will still run in the context of old build.
To get rid of this issue,we have to just restart one service and to get that service go to system services (type services.msc from run) then search for Sharepoint 2010 Timer service if the environment is Sharepoint 2010 or Windows Sharepoint Services Timer if the environment is Moss 2007.Restart the appropriate service based on the environment then the timer job will start run in the context of your latest changes to your solution.