Use JavaScript to check if a date is currently DST

is-daylight-savings.js

/**
 * Daylight saving time then ends on the first Sunday in November,
 * when clocks are moved back an hour at 2 a.m. local daylight time
 * (so they will then read 1 a.m. local standard time). In 2021,
 * DST begins on March 14 and ends on Nov. 7 in the U.S.
 *
 * Defaults to current date. Options are: `{ year: null, month: null, day: null }`
 **/
const isDST = (options = { year: null, month: null, day: null }) => {
  const d = new Date();
  const currentYear = d.getFullYear();
  const currentMonth = d.getMonth();
  const currentDay = d.getDate();

  const targetYear = options.year || currentYear;
  const targetMonth = options.month || currentMonth;
  const targetDay = options.day || currentDay;
  const currentDate = new Date(Date.UTC(targetYear, targetMonth, targetDay));
  console.log(`\nisDST() currentDate: ${currentDate}`);
  // console.log(`currentMonth: ${currentMonth}`);
  // console.log(`targetDay: ${targetDay}`);

  // all dates in April - Oct is DST
  if (targetMonth > 2 && targetMonth < 10) {
    return true;
  }

  // all dates from Dec - Feb is NOT DST
  if (targetMonth > 10 || targetMonth < 2) {
    return false;
  }

  // ..is month of November
  else if (targetMonth === 10) {
    let endDST = 1;
    for (let day = 1; day < 20; day++) {
      const novDate = new Date(Date.UTC(targetYear, 10, day)); // November
      const dayOfWeek = novDate.getDay(); // returns day-of-week number (e.g. 2 == "Tuesday")

      // 0 == "Sunday"
      if (dayOfWeek === 0) {
        endDST = day;
        break;
      }
    }

    // console.log(`endDST: ${endDST}`);
    if (targetDay >= endDST) {
      return false; // is NOT DST if after first Sun in Nov
    } else {
      return true;
    }
  }

  // ..is month of March
  else if (targetMonth === 2) {
    let startDst = 1;
    for (let day = 1; day < 32; day++) {
      const marchDate = new Date(Date.UTC(targetYear, 2, day)); // March
      const dayOfWeek = marchDate.getDay(); // returns day-of-week number (e.g. 2 == "Tuesday")

      // 0 == "Sunday"
      if (dayOfWeek === 0) {
        startDst = day + 7; // needs to be 2nd Sunday of the month
        break;
      }
    }

    // console.log(`startDst: ${startDst}`);
    if (targetDay >= startDst) {
      return true; // is DST if mid-March or later
    } else {
      return false;
    }
  }
};