A good software to slow down timelapses ?

Hello everyone, there are some very good timelapses on Youtube, and I find them as the best material for improvement. However, I have to slow down most of them to 0.25 and it is not even enough. Anyone know a good software which I can use to slow then down and render to the original lenght ? Also, how do I find out how many times the video is speeded-up ? https://www.youtube.com/watch?v=_8z2u3TUqSs This one is for example so good, but too fast. I dont want to share it anywhere, just for personal usage. Thank you.