If you specify your baseline date as yyyy-mm-dd (e.g. today is 2016-09-09), the relative dates should always work.
Some other date formats are harder for OmniFocus to parse, and it can end up not understanding where the baseline date ends and the relative date starts. If you’re having trouble with another date format, we’ll need to know exactly what you’re entering and might also need to know your current locale’s date format settings.
Yes, 2.16 added support for this (as noted over in the implementation details thread):
