Checked against a primary source — the instrument itself, not a report of it.
Verified is the top grade on our assurance ladder. It means somebody opened the primary source: a gazette, a statute, a central bank notice, a ministerial decree, and read the date there.
It does not mean the date is certain in every sense. A verified date can still be an estimate — an Islamic date read from a ministry's own published calendar is verified as to sourcing and estimated as to precision, because the ministry is itself predicting a sighting.
Filtering to verified is the right default for anything where being wrong is expensive. It will return less than the full corpus, and what it returns is what an authority actually said.
Verified means the primary source was read. Corroborated means two independent secondary sources agree, which is weaker.
Verified is about our sourcing. Precision is about the date. A verified date may still be an estimate.
See also assurance · corroborated · compiled · precision
We hold this as assurance: verified — filterable with min_assurance=verified. See what we hold.