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.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.
Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by changes in weather, the hours of daylight, and, consequently, vegetation and soil fertility.
Both *yeh₁-ro- and *h₂et-no- are based on verbal roots expressing movement, *h₁ey- and *h₂et- respectively, both meaning "to go" generally (compare Vedic Sanskrit éti "goes", atasi "thou goest, wanderest").
Derived from Latin annus are a number of English words, such as annual, annuity, anniversary, etc.; per annum means "each year", anno Domini means "in the year of the Lord".
Dates in this era are designated Anno Domini (Latin for in the year of the Lord), abbreviated AD, or the secular common era, abbreviated CE.
The year before 1 AD, or 1 CE, is designated 1 Before Christ (BC), or 1 Before the Common Era (BCE), and the year before that is 2 BC/2 BCE, etc.; hence, there was no year 0 AD/0 CE.A leap year occurs every fourth year, or leap year, during which a leap day is intercalated into the month of February. The Gregorian calendar attempts to cause the northward equinox to fall on or shortly before March 21 and hence it follows the northward equinox year, or tropical year. It is estimated that by the year 4000 CE, the northward equinox will fall back by one day in the Gregorian calendar, not because of this difference, but due to the slowing of the Earth's rotation and the associated lengthening of the day.