Gaps and overlaps
When clocks go forward, a range of local times is skipped: that day is shorter than 24 hours and those times cannot be converted without a rule. When clocks go back, a range repeats: that day is longer and each time in the range maps to two instants.
Transitions are found by scanning the browser’s rules and narrowing to the exact millisecond, so unusual changes — Lord Howe’s 30-minute shift, Morocco’s Ramadan suspensions, one-off political changes — appear as they are in the data.
Past and future years
Historic years depend on how much history the browser’s tz data keeps (most keep full IANA history back to the 1970s and beyond). Future years show today’s rules; a government can change them later.