Fun with proxies

dateUTC.js

function dateUTC(date, offset = 0){

  let d = new Date(date),
      adjust = (dir, item) => offset == 0 ? item : new Date(+item + offset * 60 * 1000 * -dir),
      shift = (v) => {s = adjust(1, d);
                      return v;},
      unshift = (v) => {d = adjust(-1, s);
                        return v;},
      rewrites = {
        getFullYear: () => s.getUTCFullYear(),
        getMonth: () => s.getUTCMonth(),
        getDate: () => s.getUTCDate(),
        getHours: () => s.getUTCHours(),
        getMinutes: () => s.getUTCMinutes(),
        getSeconds: () => s.getUTCSeconds(),
        getMilliseconds: () => s.getUTCMilliseconds(),
        getTimezoneOffset: () => offset == 0 ? 0 : offset,

        setFullYear: (y) => unshift(s.setUTCFullYear(y)),
        setDate: (d) => unshift(s.setUTCDate(d)),
        setMonth: (m) => unshift(s.setUTCMonth(m)),
        setHours: (h) => unshift(s.setUTCHours(h)),
        setMinutes: (m) => unshift(s.setUTCMinutes(m)),
        setSeconds: (s) => unshift(s.setUTCSeconds(s)),
        setMilliseconds: (ms) => unshift(s.setUTCMilliseconds(ms)),

        valueOf: () => d.valueOf()
      };

  shift();

  return new Proxy(d, {
    get(target, key, receiver){
      return function (...args) {
        if (rewrites[key]) {
          return rewrites[key].apply(this, args);
        }
        else {
          return target[key].apply(d, args);
        }
      };
    }
  });
}