Lord Howe: a 30-minute DST offset

Not every daylight-saving shift is a full hour. Lord Howe Island, off the east coast of Australia, uses a thirty-minute offset during its summer. A calculator that assumes sixty minutes whenever DST is detected will place this record half an hour away from where it belongs.

Half an hour rarely matters in the middle of a two-hour branch, which is exactly why the bug is easy to ship and hard to notice. It matters enormously when the record sits near a pillar boundary, because then thirty minutes is the whole difference between one hour pillar and the next.

This case is included because it is the kind of assumption that survives testing against comfortable data. Reading it, the useful question is not whether you were born on Lord Howe Island, but whether the tool you used has ever been checked against a place whose rules are unusual.

The general form of the problem is wider than daylight saving. Any place where the civil clock has been set by political decision rather than by geography can carry a surprise: a half-hour offset, a date line moved for trade, a wartime clock kept after the war. The cases in this category are samples of that class, not a complete catalogue of it.