See D19780. We have a reasonable amount of `1000000 * ($t_end - $t_start)` stuff in the codebase that we could simplify. In particular, it would be easy to have too many or too few `0`s somewhere and escape notice.

I landed on this method signature instead of `phutil_microseconds_between(x, y)` since I think it would be hard to remember if parameter order is `start, end` (chronological) or `end, start` (like arithmetic subtraction). It should be a little simpler to use in most cases, too.