Returns the number of years between two dates.


YEARS(startdate, enddate, mode)
If mode is 0, YEARS returns the number of whole years between startdate and enddate, day/month to day/month. 
If mode is 1, YEARS identifies the year that startdate and enddate each lie in, and returns the difference between those years. In other words it returns YEAR(enddate) - YEAR(startdate)
If startdate is after enddate the result will be negative.


YEARS("2009-04-03", "2011-11-17", 0)
returns 2.
YEARS("2011-02-28", "2012-02-28", 0)
returns 1. 2012 is a leap year, but 28th February to 28th February is classed as a whole year.
YEARS("2012-02-29", "2013-02-28", 0)
returns 0. 2012 is a leap year, but 29th February to 28th February is not classed as a whole year, even though both dates are the last day of February.
YEARS("2009-04-03", "2011-11-17", 1)
returns 2.
YEARS("2009-12-31", "2010-01-01", 1)
returns 1. Even though there is only one day between the dates, they lie in different years.

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