The first entry is always the pin's source. Overall confidence is a weighted average of how firmly each reference supports the start and end times used above; a reference counts half as much for every 180 days older than the newest.
Happened as dated, per the source: "The Jubilee line platforms were opened on 22 December 1999."
Bechtel's own project page confirms it delivered the Jubilee Line Extension, including the rebuilt Westminster station, ahead of the December 1999 deadline, but does not itself give the Westminster platforms' exact opening day.
Wikipedia confirms "the Jubilee line platforms were opened on 22 December 1999" at Westminster station, matching the pin's stored date exactly.
Something missing or wrong? Say it in your own words: a link that backs this pin up, a different start or end date and why, or a fact it lacks or gets wrong. The AI checks it against this pin's sources, searches for better ones, and adds any page that backs you up. The pin's own sources still count most. A picture that shows something else, or shows it badly, is looked at too, and moved down or replaced.