The rainbow timer mod adds a timer in rainbow colors in your actionbar that is customizable trough commands. There are five commands that you can use:
/timer resume (starts/ resumes timer)
/timer pause (pauses timer)
/timer set [x] [x] [x] [x] sets the timer to [x] days, hours, minutes and seconds
/timer reverse - will be available soon
/timer hide (Hides the timer)
/timer show (Makes the timer visible)