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 arstechnica.com: "…nd keyboard-friendly Windows 8.1 Update arrives April 8 Microsoft makes yet more adjustments to Wind…"
TechCrunch reports Microsoft unveiled Windows 8.1 Update 1 at its April 2, 2014 Build conference, stating the update 'will begin to roll out via Windows Update on April 8th' - matching the pin's stored April 7-9, 2014 window.
Fox News similarly reports the update was announced at Build 2014 and slated for release as a free update on April 8, 2014, Microsoft's next Patch Tuesday, corroborating the pin's stored date.
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.