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: "officially opened by Queen Elizabeth II accompanied by Prince Philip, Duke of Edinburgh, on 14 July 1994".
Wikipedia states the SIS Building "was completed in April 1994 and officially opened by Queen Elizabeth II... on 14 July 1994," matching the pin's stored date exactly.
Britannica's own entry on the SIS Building is a respected general reference, though the page itself could not be fetched to quote directly.
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.