public long getOffsetMillis(final UTCDate value) {
        final String offsetString = getOffsetString(value);
        int hours = ClientStringUtil.getInt(offsetString.substring(1, 3));
        int minutes = ClientStringUtil.getInt(offsetString.substring(3, 5));
        long millis = (hours * MILLIS_IN_HOUR) + (minutes * MILLIS_IN_MINUTE);
        if (offsetString.charAt(0) == '-') {
            millis = millis * -1;
        }
        return millis;
    }
Shifts the currently specified date by the given number of months. The day
of the month will be pinned to the original value as far as possible.

@param deltaMonths - number of months to be added to the current date
stroom-core-client-widget/src/main/java/stroom/widget/datepicker/client/DateTimeModel.java