`Date.now()` isn't introduced into IE until version 10, so we can't use it, which is unfortunate, since not only is it nicer to read, it's evidently the fastest:

http://jsperf.com/new-date-vs-date-now-vs-performance-now/6

I'll go with `(new Date()).getTime()` which appears to be slightly faster than `+new Date()`, and also more readable.