Couple notes here, all of which apply to the entire PR - 

* Wondering if, at least on these stand-alone functions, we should have the return value filtered?  I know we can just filter on get_option, but it might be nice to have one here.  Open to other thoughts.
* Since 3.5 is now our minimum, it'd be nice to use the constants provided by WP here (HOUR_IN_SECONDS, DAY_IN_SECONDS, WEEK_IN_SECONDS)
* WordPress Coding Standards dictate that when a dynamic key is being selected from an array, breathing room is required.  E.g. `$convert[ $interval ]` instead of `$convert[$interval]`