为什么我尝试用Web性能API来度量渲染时间，结果总是为负值

I'm building a web-based page render benchmark app which uses the Web Performance API. What it does is basically measuring how much time the browser takes when ordered to render a bunch of div elements, and then build a hidden form, put the calculation result in there, and submit the form to another PHP page for processing. The problem is, the calculation result keeps being negative and in the several trillion range (i.e. a fourteen digit number: -1364403484035). Here's the code:I'm building a web-based page render benchmark