Convert Unix timestamps

Convert a timestamp to a readable date in the timezone you choose, or a date back to a timestamp, in seconds or milliseconds. Two things this gets right that are easy to get wrong: dates before 1970 are shown as the negative timestamps they are rather than treated as errors, and a wall-clock time that daylight saving makes ambiguous or impossible is flagged instead of quietly resolved — which is how a scheduled job ends up an hour out twice a year.