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In tropical and subtropical regions several geographical sectors do not present defined seasons; but in the seasonal tropics, the annual wet and dry seasons are recognized and tracked. A calendar year is an approximation of the number of days of the Earth's orbital period as counted in a given calendar.
The Gregorian, or modern, calendar, presents its calendar year to be either a common year of 365 days or a leap year of 366 days, as do the Julian calendars; see below.
For the Gregorian calendar the average length of the calendar year (the mean year) across the complete leap cycle of 400 years is 365.2425 days.
No astronomical year has an integer number of days or lunar months, so any calendar that follows an astronomical year must have a system of intercalation such as leap years.
Financial and scientific calculations often use a 365-day calendar to simplify daily rates.
In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.
Similarly, "year" can mean the orbital period of any planet: for example, a Martian year or a Venusian year are examples of the time a planet takes to transit one complete orbit.
In some languages, it is common to count years by referencing to one season, as in "summers", or "winters", or "harvests".As 218 out of every 900 years are leap years, the average (mean) length of this Julian year is A calendar era assigns a cardinal number to each sequential year, using a reference point in the past as the beginning of the era.Worldwide, the most commonly used calendar era is referenced from the traditional—now believed incorrect—year of the birth of Jesus.The Persian calendar, in use in Afghanistan and Iran, has its year begin at the midnight closest to the instant of the northward equinox as determined by astronomical computation (for the time zone of Tehran), as opposed to using an algorithmic system of leap years.A fiscal year or financial year is a 12-month period used for calculating annual financial statements in businesses and other organizations.In temperate and subpolar regions around the planet, four seasons are generally recognized: spring, summer, autumn and winter.