"Next <weekday>" is sometimes expressed as an absolute date

[gcampax]

The (English postprocessing) NLG code recognizes that if the date is less than a week apart, we should say "next " (next Monday, next Tuesday, etc.). This logic seems to be broken when the date is exactly one week apart. That is, on Sunday, when talking about "next Sunday at XX:XX am" , we say an absolute date ("on September 19 at XX:XX").

This is causing https://github.com/stanford-oval/thingpedia-common-devices/issues/431 which makes the scenario tests fail intermittently.