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 0s 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.