Time and Space Machine
Perhaps you haven't thought about it, but a time machine should move a person not only in time but also in space, taking into account the position of the Earth.
Because if you go back a couple of months to the same point in the universe, you will simply die, suspended in space.
And if the universe itself is not stationary and is moving within another space that we do not know about, then you can die in about 100% of travel cases, no matter what you've calculated.
Have a nice day!