src/modules/add-time-or-subtract-time.js
function addTimeOrSubtractTime(increment, n, date) {
  const incrementHandlers = {
    [MILLISECOND](date) {
      return new Date(date.setUTCMilliseconds(date.getUTCMilliseconds() + n));
    },

    [SECOND](date) {
      return new Date(date.setUTCSeconds(date.getUTCSeconds() + n));
    },

    [MINUTE](date) {
      return new Date(date.setUTCMinutes(date.getUTCMinutes() + n));
    },

    [HOUR](date) {
      return new Date(date.setUTCHours(date.getUTCHours() + n));
    },

    [DATE](date) {
      return new Date(date.setUTCDate(date.getUTCDate() + n));
    },

    [WEEK](date) {
      return new Date(date.setUTCDate(date.getUTCDate() + n * 7));
    },

    [MONTH](date) {
      let newMonth = date.getUTCMonth() + n;
      let newYear = date.getUTCFullYear();
      let newDate = date.getUTCDate();

      if (newDate > new Date(date.setUTCFullYear(newYear, newMonth + 1, 0)).getUTCDate()) {
        return new Date(date.setUTCFullYear(newYear, newMonth + 1, 0));
      }
      return new Date(date.setUTCFullYear(newYear, newMonth, newDate));
    },

    [YEAR](date) {
      let newYear = date.getUTCFullYear() + n;
      let newMonth = date.getUTCMonth();
      let newDate = date.getUTCDate();

      if (newDate > new Date(date.setUTCFullYear(newYear, newMonth + 1, 0)).getUTCDate()) {
        return new Date(date.setUTCFullYear(newYear, newMonth + 1, 0));
      }
      return new Date(date.setUTCFullYear(newYear, newMonth, newDate));
    },
  };

  return incrementHandlers[increment](date);
}
function addTimeOrSubtractTime(increment, n, date) {
  const incrementHandlers = {
    [MILLISECOND](date) {
      return new Date(date.setUTCMilliseconds(date.getUTCMilliseconds() + n));
    },

    [SECOND](date) {
      return new Date(date.setUTCSeconds(date.getUTCSeconds() + n));
    },

    [MINUTE](date) {
      return new Date(date.setUTCMinutes(date.getUTCMinutes() + n));
    },

    [HOUR](date) {
      return new Date(date.setUTCHours(date.getUTCHours() + n));
    },

    [DATE](date) {
      return new Date(date.setUTCDate(date.getUTCDate() + n));
    },

    [WEEK](date) {
      return new Date(date.setUTCDate(date.getUTCDate() + n * 7));
    },

    [MONTH](date) {
      let newMonth = date.getUTCMonth() + n;
      let newYear = date.getUTCFullYear();
      let newDate = date.getUTCDate();

      if (newDate > new Date(date.setUTCFullYear(newYear, newMonth + 1, 0)).getUTCDate()) {
        return new Date(date.setUTCFullYear(newYear, newMonth + 1, 0));
      }
      return new Date(date.setUTCFullYear(newYear, newMonth, newDate));
    },

    [YEAR](date) {
      let newYear = date.getUTCFullYear() + n;
      let newMonth = date.getUTCMonth();
      let newDate = date.getUTCDate();

      if (newDate > new Date(date.setUTCFullYear(newYear, newMonth + 1, 0)).getUTCDate()) {
        return new Date(date.setUTCFullYear(newYear, newMonth + 1, 0));
      }
      return new Date(date.setUTCFullYear(newYear, newMonth, newDate));
    },
  };

  return incrementHandlers[increment](date);
}
Adds or subtracts specified increments to or from a date object
@param   {String}  increment   an increment to add
@param   {Number}  n           factor to add or subtract the increment by
@param   {Date}    date        date object
@returns {Date}                a new date