The Doctor would not be crossing his own time line in this case. He just go in the past to all points nowhere, for whatever time he needed to finish his translator*, and come back a second after his departure. The problem come with trusting the Tardis to do it right. Also you could say, the Doctor has the bad habit of getting side tracked. *The gizmo, I don't know how he call it.
from overview for madhi19 http://ift.tt/10cgHwC