Added time constants

*Do not merge*

@jkburges a few times the idea of using time constants has come up in reviews. Just wanted to get your opinion on this scheme of constants (I haven't written unit tests, yet). I just want to make sure we have something that will read well and is easy to use. My only worry with this is that it assumes a base unit of milliseconds so if a function needs seconds, minutes, etc. then the developer will have to identify that. I have ideas about how we could tackle that when it comes up.