Basic Concept

• Gregorian /ɡri'ɡɔ:riən/ 格利高里圣歌 提供了世界上大多数国家/地区使用的标准日历系统
• Date.UTC(1970, 0, 1, 0, 0, 0) == 0
• GMT vs UTC

Java Source Code Analysis

  /**
  *  @param millis,  the time between the 1970, jan1
  */
  public CalendarDate getCalendarDate(long millis, CalendarDate date) {
        int ms = 0;             // time of day, to calculate the hour, minute, second, millis
        int zoneOffset = 0;
        int saving = 0;
        long days = 0;          // fixed date

        // adjust to local time if `date' has time zone.
        TimeZone zi = date.getZone();
        if (zi != null) {
            int[] offsets = new int[2];
            if (zi instanceof ZoneInfo) {
                zoneOffset = ((ZoneInfo)zi).getOffsets(millis, offsets);
            } else {
                zoneOffset = zi.getOffset(millis);
                offsets[0] = zi.getRawOffset();
                offsets[1] = zoneOffset - offsets[0];
            }

            // We need to calculate the given millis and time zone
            // offset separately for java.util.GregorianCalendar
            // compatibility. (i.e., millis + zoneOffset could cause
            // overflow or underflow, which must be avoided.) Usually
            // days should be 0 and ms is in the range of -13:00 to
            // +14:00. However, we need to deal with extreme cases.
            days = zoneOffset / DAY_IN_MILLIS;
            ms = zoneOffset % DAY_IN_MILLIS;
            saving = offsets[1];
        }
        date.setZoneOffset(zoneOffset);
        date.setDaylightSaving(saving);

        days += millis / DAY_IN_MILLIS;
        ms += (int) (millis % DAY_IN_MILLIS);
        if (ms >= DAY_IN_MILLIS) {
            // at most ms is (DAY_IN_MILLIS - 1) * 2.
            ms -= DAY_IN_MILLIS;
            ++days;
        } else {
            // at most ms is (1 - DAY_IN_MILLIS) * 2. Adding one
            // DAY_IN_MILLIS results in still negative.
            while (ms < 0) {
                ms += DAY_IN_MILLIS;
                --days;
            }
        }

        // convert to fixed date (offset from Jan. 1, 1 (Gregorian))
        days += EPOCH_OFFSET;

        // calculate date fields from the fixed date
        getCalendarDateFromFixedDate(date, days);

        // calculate time fields from the time of day
        setTimeOfDay(date, ms);
        date.setLeapYear(isLeapYear(date));
        date.setNormalized(true);
        return date;
    }

    // The number of days between January 1, 1 and January 1, 1970 (Gregorian)
    static final int EPOCH_OFFSET = 719163;

Reference

GregorianCalendar 标准阳历
JavaScript Date