Given a time value, it is helpful to be able to compute a year number from it. The ECMA compliant implementations use the extrapolated Gregorian system to map days to years and compute a time relative to the reference date of 01-January-1970 UTC. From there the year number can be established.

All non-leap years have 365 days with the usual number of days in each month. Leap years have an extra day in February. The calculation shown below uses known leap years and non-leap years to adjust the day numbers and yield the day number of the first day of the given year, and then use that to work out the time in milliseconds when the year started:

   DayFromYear(y) =

   365 * (y - 1970) +

   floor((y - 1969) / 4) -

   floor((y - 1901) / 100) +

   floor((y - 1601) / 400)

   msPerDay = 86400000

   

   TimeFromYear(y) = msPerDay * DayFromYear(y)

YearFromTime(t) = The largest integer y to make TimeFromYear(y) less than or equal to t.

Example code:

   // Work out year number from time

   var msPerDay = 86400000;

   var myMilliseconds = Number(new Date());

   document.write(yearFromTime(myMilliseconds));

   // Return year number based on time value

   function yearFromTime(aMilliseconds)

   {

   var myStartYear = 1970;

   

   while(timeFromYear(myStartYear) < myMilliseconds)

   {

   myStartYear++

   }

   

   return myStartYear-1;

   }

   // Work out milliseconds at start of year

   function timeFromYear(aYear)

   {

   var myTime = msPerDay * dayFromYear(aYear);

   return myTime;

   }

   // Work out day number from milliseconds

   function dayNumber(aMillisecondTime)

   {

   var myDay = Math.floor(aMillisecondTime/msPerDay);

   

   return myDay;

   }

   // Day from year function

   function dayFromYear(aYear)

   {

   var myDay = 365 * (aYear - 1970)             +

   Math.floor((aYear - 1969) / 4)   -

   Math.floor((aYear - 1901) / 100) +

   Math.floor((aYear - 1601) / 400);

   return myDay;

   }

Cross-references:

ECMA 262 edition 2 - section - 15.9.1.3

ECMA 262 edition 3 - section - 15.9.1.3