Where the dates come from
Every claim here is two dates and a subtraction, so the dates had better be right. This page says where they come from and what happens when one is not.
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 is why it is used here. A date read from a field is unambiguous in a way that a date read out of a paragraph is not, and every entry links back to where it came from, so any claim on this site can be traced to its source in a click.
What is checked
Dates are verified before anything is published. A date has to be precise to the year, so a subject known only to a century does not appear. Where sources disagree with each other about a date, the subject is left out rather than picked between. And the sentence a page makes has to be true of the subjects it names, which is checked rather than assumed.
Each subject on a page links to its own record, so none of this has to be taken on trust.
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.