Where the dates come from
Every comparison here depends on dates, so those dates and what they measure have to be clear. This page says where they come from, how uncertain dates are shown and what happens when one is wrong.
Wikidata
Wikidata is a free, openly edited database of facts that sits alongside Wikipedia. Where a Wikipedia article is prose, a Wikidata entry is structured: a record for Cleopatra holds her date of birth and date of death as separate, sourced fields rather than as a sentence.
That structure makes Wikidata useful for finding candidate dates. Published explanations also link the claim to a suitable museum, archive, library or scholarly source, so the wording can be checked against the evidence that supports it.
What is checked
Dates are verified before anything is published. A date used as the correct answer in the game has to be precise to the year. An explanatory event may be shown as a range when the evidence supports only that range. The drawing keeps the full range visible, and the page makes only comparisons that stay true across every supported year.
When sources disagree, their combined range and disagreement are stated. If the conflict cannot support one honest range or changes the conclusion, the claim is left out. Each explanation links to its sources, so none of this has to be taken on trust.
There was no year zero: 1 BC was followed by AD 1. Every span on this site is counted that way, so the years from 30 BC to AD 1969 are 1,998, not the 1,999 a plain subtraction gives.
When we get something wrong
Facts get revised. When a date changes, the item comes out of rotation and its page carries a visible correction saying what changed. Corrections are not made quietly, because a page that silently becomes right is indistinguishable from one that was never wrong.
The crowd figures
Guesses are stored as simple counts. There is no record of who guessed what: a guess adds one to a tally and is from then on indistinguishable from every other guess beside it.
A crowd figure appears only once an item has at least 50 guesses, because a median drawn from a handful of people would not mean anything.
The people who play this are people who chose to play a game about time. Read the crowd marker as what history enthusiasts think, not as what everyone thinks.