When two black-holes merge, they 'ringdown' as they settle into a final Kerr black hole. The ringdown part of the gravitational wave signal probes the strong field gravity, enabling us to test the general theory of relativity (GR) in that regime. In this talk, I will summarize our prospects to perform the these GR tests with the ringdown signals using the current and future ground based gravitational wave observatory. I will also introduce the challenges we face in the data analysis of these signals and review its current status. Then, I will focus on a particular problem with these signals - estimating the start time of the ringdown. I will discuss this, both from the point of view of the dynamics observed in the source frame and the analysis on gravitational wave done in the asymptotic frame.