PROBLEM SOLVED - EXPLANATION BELOW.

I'm working on a graphing function which calculates the min/max values of the graph. I want to be able to take the min/max and extend them a small degree so that there is a small amount of padding on the left of the min and the right of the max. I want to increase/decrease the date by 10% of the difference of the two dates. So if the dates are separated by 10 days I want to add 1 day to the max and subtract 1 day from the minimum.

This seems like it would be an easy task but I can't seem to get it right.
Code JavaScript:
var max = 1269003636000;
var min = 1011020436000;
var constant = 0.1;
 
var offSet = Math.abs((max - min)*constant)
max += offSet;
min -= offset;

What changes should I make to get this to work properly?

EDIT
____________________________________________________

It did turn out to be something simple. The var min/max were being read as strings so += was concatenating instead of adding. A simple use of parseInt solved the problem.