Fixes bug 1023867 - stop treating negative install_age as valid

Found an out-of-bounds bug in a tiny helper function, tucked inside of our duplicate finder script. 
This patch eliminates the treatment of negative `install_age` as a valid value for the check to see whether a timestamp is "approximately" the same as another timestamp, and simply returns `False` when a negative integer value is encountered. 
this is the relevant change