In fact, it used to be that all years were defined to have 365.0 days, and the calendar drifted away from the true seasons as a result. In the year 46 BCE, Julius Caeser established the Julian Calendar, which implemented the world's first leap years: He decreed that every 4th year would be 366 days long, so that a year was 365.25 days long, on average. This basically solved the calendar drift problem.
De feito, antes todos os anos definíanse como de 360, 5 días e, como consecuencia, o calendario derivaba das estacións reais. No ano 46 AdC Xulio César estableceu o calendario xuliano, que incluía os primeiros anos bisextos do mundo. Decretou que un ano de cada catro tivese 366 días para que un ano tivese 365, 25 días de media. Isto resolveu basicamente o problema da deriva do calendario.