Having an exact difference value between two Date value

Having an exact difference value between two Date value

I want to do somthing like this:
Date startDate = new Date();
// Do Something
Date endDate = new Date();
double msElapsedTime =endDate.getTime()-startDate.getTime();
I have the result value in milisecond like this:7.0 but I want to have the
exaxt value for example:7.15
How can I do that?