Some calendar functions

calendar-util.js

// calculate week number for date

function calcWN(y, m, d) {
  let J = gc2jd(y, m, d);
  let d4 = (J + 31741 - J % 7) % 146097 % 36524 % 1461;
  let L = 0 | d4 / 1460;
  let d1 = (d4 - L) % 365 + L;
  return (0 | d1 / 7) + 1;
}

// calculate week day for date
// 0 - sunday, 6 - saturday
function calcWD(y, m, d){
  return (d += m < 3 ? y-- : y - 2, (0 | 23 * m / 9) + d + 4 + (0 | y / 4) - (0 | y / 100) + (0 | y / 400)) % 7;
}

// convert date to julian day
function gc2jd(y, m, d) {
   [y, m] = m < 3 ? [y - 1, m + 12] :[y, m];
   return 2 - (0 | y / 100) + (0 | y / 400) + d + (0 | 365.25 * (y + 4716)) + (0 | 30.6001 * (m + 1)) - 1524;
}

// convert julian day to date
function jd2gc(J) {
  let A = J + 32044;
  let B = (0 | (4 * A + 3) / 146097);
  let C = A - (0 | (B * 146097) / 4);
  let D = (0 | (4 * C + 3) / 1461);
  let E = C - (0 | (1461 * D) / 4);
  let M = (0 | (5 * E + 2) / 153);
  let day = E - (0 | (153 * M + 2) / 5) + 1;
  let month = M + 3 - 12 * (0 | (M / 10));
  let year = B * 100 + D - 4800 + (0 | (M / 10));
  return { year, month, day };
}


// time shift for julian day
function time2jt(hours, min, sec, tz) {
  let pds = 864e2;
  return ((hours - tz) * 3600 + min * 60 + sec) / pds;
}

// convert timestamp (in seconds) to julian day
function ts2jd(ts) {
  let origin = 2440587.5;
  let pds = 864e2;
  return origin + (ts / pds);
}

// convert julian day to timestamp (in seconds)
function jd2ts(jd) {
  let origin = 2440587.5;
  let pds = 864e2;
  return (jd - origin) * pds;
}