Adventures in Daylight Saving, Norfolk Island, and Time Zone Math (in Ruby)

distance_of_time_in_words is a small Ruby gem that turns two Time objects into a human-readable string like “3 days and 4 hours”. Two separate bug reports against it this year turned out to be variations on the same theme: computing a duration between two timestamps is not the trivial subtraction it looks like, the moment time zones are involved. Both fixes shipped in dotiw 5.6.0. Bug 1: dst? Lies When You Least Expect It #63 reported that a duration of one minute was rendered as “less than 1 second” for users in Europe/Dublin. The gem’s TimeHash had a DST correction that looked reasonable: d = largest - smallest d -= 1.hour if smallest.dst? && !largest.dst? d += 1.hour if !smallest.dst? && largest.dst? The idea: if a DST transition happened between the two times, Time subtraction already accounts for the wall-clock jump, so cancel it back out before splitting the duration into calendar units. That works everywhere except Ireland. Europe/Dublin uses an inverted DST scheme: its winter…