Atul,
You have referred to pratyantar dasha in your post. Dasha calculations (as well as navamsha) are very sensitive to birth time. An error of a few minutes can change the dasha calculations, as well as the navamsha chart.
In 1968, when I was born, clocks were mostly mechanical, and they lost time. The only public medium was radio. Calibration of clocks was probably not done regularly. So I don't know how accurate a dasha calculation will be in my case, or for anyone born before the 1990s.