fisrt of all it would be very stupid to make the gate go back in time and not allow it to travel to another point in space at teh same time, and also if the gate had a atime travel funtion on it than why (WARNING STARGATE ATLANTIS THE GIFT)
they did this because the ancients knew that the effects of time travel could be potentially diasterous on the space time continum, but lets say for the fact of argument that the ancients didnt realize the effects that time travel can have (see grandfather paradox etc.) when they put the function on the gate, then why would they leave the ninth chevrons on all of the stargates leaving the time travel fuction potentialy still activated, leaving a function that could posibly desstroy the universe as we know it on the piece of technology that is used most often in our galaxy, answer that you dont even need to read the spoilers for this to make sesnse
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