The lifeclock is the crystal in the palm of every citizen's right hand.
In The Novel, a person's age is revealed by their palm flower crystal embedded in the palm of their left hand that changes color every seven years, yellow (age 0-6), then blue (age 7-13), then red (age 14-20), then blinks red and black on Lastday, and finally turns black at 21.
In The Movie, the crystal is the right hand, newborns have a clear white crystal, which stays clear until the age of 9, then it turns yellow. At age 16 it turns green until age 21-24 when it turns red. Towards last day, the red blinks black. The lifeclocks of those who venture outside the city turn clear white.