Era Sputnik (abbreviated ES) is the standard year numbering system introduced in Last Order, which is set in ES 591. It superseded the prior dating convention of Anno Koyomi, which used the years before or after Koyomi's birth.
The Era Sputnik system was conceived by Arthur Farrell following the defeat of the Type-V mutants near the former St. Louis, Missouri and the end of the decades-long impact winter. The first year or ES 1 was set as 1957, the year the first artificial satellite, Sputnik, was launched into space. Its calendar system is the same as a standard Earth revolution around the Sun. On Ketheres there is a small monument dedicated to Kudryavka, the first living mammal from Earth to reach Earth's orbit.