function fromMillis(millis, picture, timezone) {
        // undefined inputs always return undefined
        if(typeof millis === 'undefined') {
            return undefined;
        }

        return formatDateTime.call(this, millis, picture, timezone);
    }
Converts milliseconds since the epoch to an ISO 8601 timestamp
@param {Number} millis - milliseconds since the epoch to be converted
@param {string} [picture] - the picture string defining the format of the timestamp (defaults to ISO 8601)
@param {string} [timezone] - the timezone to format the timestamp in (defaults to UTC)
@returns {String} - the formatted timestamp