In the Gregorian calendar, a year ending in “00” that is divisible by 400 is a **century leap year**, with the intercalation of February 29 yielding 366 days instead of 365. Century years (divisible by 100) that are *not* divisible by 400 are common years (with 365 days) and not leap years. For example, the years 1600, 2000, and 2400 are century leap years since those numbers are divisible by 400, while 1700 liter water bottle bpa free, 1800, 1900, 2100, 2200, and 2300 are common years despite being divisible by 4. Leap years divisible by 400 always start on a Saturday; thus the leap day February 29 in those years always falls on a Tuesday (dominical letter BA).

The Gregorian calendar yields an average year that currently tracks the annual revolution period of the Earth more closely than the older Julian calendar, in which every fourth year (including end-of-century years) is a leap year. The Julian formula adds too many leap days (3 every 400 years), causing the Julian calendar to drift gradually with respect to the astronomical seasons. Over time, natural events such as the spring equinox began to occur earlier and earlier in the Julian calendar.

The Gregorian calendar was introduced in 1582, but was adopted by various countries over several centuries, with the result that some countries still used the older Julian calendar while others used the Gregorian calendar. Dates prior to 1582 are generally calculated using the Julian calendar, and different countries have different conventions about dates between 1582 and their adoption of the Gregorian calendar. (See, for example, Old Style and New Style dates reusable glass bottles.)