From the "Appendix / methodology" section at the bottom of the page: > "Day 1" is when a country reaches 100 cases. A country is "X days behind Italy" if they

From the "Appendix / methodology" section at the bottom of the page: > "Day 1" is when a country reaches 100 cases. A country is "X days behind Italy" if they reached 100 cases that many days after Italy did.