Seconds pendulums were used in vintage clocks which advanced one second, every time the pendulum reached an extreme position.A seconds pendulum takes exactly 1 second to move from one extreme pendulum takes 2 second to complete one ossiliation or the time period of a seconds pendulum in 2 second.
Since the pendulum reaches its extreme position twice in one oscillation, the time period of a seconds pendulum is 2 seconds.
The time period of a simple pendulum of length L is
T=2π√(l/g)
Put T=2 seconds
g = 9.81m/s^2
L=g/π^2=.994m
On earth, the seconds pendulum has a constant length of approximately 1m