As promised, this fixes the edge case identified in the post-close comments on Pull Request #867 - to reiterate:
"Ok, so we (internally) identified a minor issue with this pull request, and that is in the edge case where the elapsed duration is exactly one second. The way the translation string is evaluated, it will try to find a string that terminates with "secondAgo" and or "secondIn", neither of which currently exists in the strings file or the unit tests. I'll fix it as soon as I'm back from vacation (sunday)."
Fix was to add the secondIn and secondAgo translation strings, and to adjust the unit tests to also test that particular edge case.
Added special getter for "elapsed" time.
Created "%E" and "%e" time formatters, for elapsed time measurments.
Added datetime/core dependency to datetime/localized.
Fixed source formatting.
Moved string formatting logic into datetime/localized as it depends on
Created stub method in datetime/core
Added %E and %e to unit tests in datetime/core
Bugfix: Added an additional translation string (and appropriate unit
tests) for the one-second case that translates to _SC.DateTime.secondAgo
Thanks for staying on top of this.