yearfrac.js

import parsedate from "./parsedate";
import isleapyear from "./isleapyear";
import serial from "./serial";

export default function yearfrac(start_date, end_date, basis) {
  start_date = parsedate(start_date);
  if (start_date instanceof Error) {
    return start_date;
  }
  end_date = parsedate(end_date);
  if (end_date instanceof Error) {
    return end_date;
  }

  basis = basis || 0;
  var sd = start_date.getDate();
  var sm = start_date.getMonth() + 1;
  var sy = start_date.getFullYear();
  var ed = end_date.getDate();
  var em = end_date.getMonth() + 1;
  var ey = end_date.getFullYear();

  function isLeapYear(year) {
    return (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;
  }
  function daysBetween(a, b) {
    return serial(b) - serial(a);
  }

  switch (basis) {
    case 0:

      if (sd === 31 && ed === 31) {
        sd = 30;
        ed = 30;
      } else if (sd === 31) {
        sd = 30;
      } else if (sd === 30 && ed === 31) {
        ed = 30;
      }
      return (ed + em * 30 + ey * 360 - (sd + sm * 30 + sy * 360)) / 360;
    case 1:

      var feb29Between = function(date1, date2) {
        var year1 = date1.getFullYear();
        var mar1year1 = new Date(year1, 2, 1);
        if (isLeapYear(year1) && date1 < mar1year1 && date2 >= mar1year1) {
          return true;
        }
        var year2 = date2.getFullYear();
        var mar1year2 = new Date(year2, 2, 1);
        return isLeapYear(year2) && date2 >= mar1year2 && date1 < mar1year2;
      };
      var ylength = 365;
      if (
        sy === ey ||
        (sy + 1 === ey && (sm > em || (sm === em && sd >= ed)))
      ) {
        if (
          (sy === ey && isLeapYear(sy)) ||
          feb29Between(start_date, end_date) ||
          (em === 1 && ed === 29)
        ) {
          ylength = 366;
        }
        return daysBetween(start_date, end_date) / ylength;
      }
      var years = ey - sy + 1;
      var days =
        (new Date(ey + 1, 0, 1) - new Date(sy, 0, 1)) / 1000 / 60 / 60 / 24;
      var average = days / years;
      return daysBetween(start_date, end_date) / average;
    case 2:

      return daysBetween(start_date, end_date) / 360;
    case 3:

      return daysBetween(start_date, end_date) / 365;
    case 4:

      return (ed + em * 30 + ey * 360 - (sd + sm * 30 + sy * 360)) / 360;
  }
}