Testing running time between two javascripts

i have two java scripts running on different sites what i need to find is how much time passed between the first script starting point to the second script ending point Just few points to consider

1. the two scripts have the same name - i can't change that.

2. there are few other scripts between those two scripts - whatever the solution i need it to take the time of those scripts no matter how many scripts are running

3. the solution must not slow the scripts in any way!!!