As far as I know, the calendar most used, the Gregorian calendar, is the most accurate. It takes the number of days the Earth takes to circulate the sun and with the odd bit left over, a leap year every fourth year, brings the movement of time into a neat calculation. The Gregorian is the calendar used in the international standard for Representation of dates and times.
Because it calculates the year from the time it takes the Earth to circulate the sun, it is the most accurate way of calculating the passing of time, making sense of the term A Year. the Gregorian calendar aligns with the movement of the Earth around the Sun and also corresponds to changes in seasons at different latitudes.