The timeclock package adds up time intervals, so you can (for instance) keep track of how much time you spend working on particular projects.
Use the M-x timeclock-in command when you start working on a project, and M-x timeclock-out command when you're done. Each time you do this, it adds one time interval to the record of the project. You can change to working on a different project with M-x timeclock-change.
Once you've collected data from a number of time intervals, you can use M-x timeclock-workday-remaining to see how much time is left to work today (assuming a typical average of 8 hours a day), and M-x timeclock-when-to-leave which will calculate when you're done.
If you want Emacs to display the amount of time left of your
workday in the mode line, either customize the
timeclock-modeline-display variable and set its value to
t, or invoke the M-x timeclock-modeline-display command.
Terminating the current Emacs session might or might not mean that
you have stopped working on the project and, by default, Emacs asks
you. You can, however, customize the value of the variable
nil to avoid the question;
then, only an explicit M-x timeclock-out or M-x
timeclock-change will tell Emacs that the current interval is over.
The timeclock functions work by accumulating the data in a file
called ~/.emacs.d/timelog. You can specify a
different name for this file by customizing the variable
timeclock-file. If you edit the timeclock file manually, or if
you change the value of any of timeclock's customizable variables, you
should run the command M-x timeclock-reread-log to update the
data in Emacs from the file.