Birthday balloon pendulum

I received a helium balloon on my birthday! I found that it had a very damped oscillatory motion. I counted 5 cycles in 35 seconds. This was an occasion on which I should have used a fiducial marker. This is an indicator of the centre of the oscillation. Oscillations are timed from the fiducial marker. This is because the object is moving fastest at that point. I timed from the amplitude. That seems better but the problem then is judging the exact time at which it stops moving and turns to go back. With fast oscillations there is little difference but with slow oscillations using a fiducial marker is more accurate. So for my balloon, one time period = 7 seconds. For a normal pendulum, time period T = 2pi x square root (L/g). So if this equation applied to the balloon, L=T squared x g / 4 x pi squared, which comes to 12.4 metres. The string was less than 2 metres long so the normal pendulum equation does not apply.