Time

If you take a watch to measure time, how accurate is it? Well, no
matter how accurate it is, it will never measure time accurately
enough. This is because by the time the watch has measured time, this
continuous variable has moved forward in time!

Suppose you were a human being able to time travel and you took three
steps forward in time, then two steps backward in time. How many steps
forward would you have travelled in time?

Well it would most certainly be more than one, because as you travelled
forward in time, you travelled a little bit more, before reversing in
time. Therefore without knowing how far you have travelled in time, it
isn't possible to travel backwards in time accurately.

This is known as the x factor and bothered Einstein a lot.