v0.20.0

Fixed

• now-12:30 and similar DateTime - DateTime subtractions no longer collapse a
  negative difference to 0 seconds; the signed duration is now returned
  (issue #185).

Added

• The web app now interprets now and timezone-less times (e.g. 12:30) in the
  user's local timezone by default, so now-12:30 matches the wall clock.
  Times with an explicit timezone (e.g. 12:30 UTC) are still honored as given.
• New Calculator::set_timezone_offset(offset_minutes) and
  Calculator::clear_timezone_offset() WASM/library methods to control the
  local timezone offset used when resolving now and bare times.