Play and Play Reverse _ Timeline

Hello there,

I have attached file showing small animation:

Initially square is on top when you click red arrow, square goes down and stops; afterwords you press green arrow, square goes up and stays. Problem is if you press green arrow twice subsequently id does funny thing, same as with red arrow. I achieved this with time line, one straight animation, other butted, reverse. Is there a way to press geen arrow, and while is pressed, square goes down, but if you release arrow before it riches down it stops. Now, if you press green arrow again, continues to the bottom, and stops, although you are pressing; at the same time, if square was on the way down, and you release arrow, and this time press red arrow, it goes up and when it riches the top it stops.
Please if anyone can help with some code, idea, tut, incaridgement, like, you can do it :yarr: or some form of help.

Thanks