@Override
    public String toString() {
        StringBuilder ser = new StringBuilder();
        /** convert long value to string */
        String dateTime = Long.toString(refSecondsSinceEpoch);
        ser.append(dateTime);
        return ser.toString().trim();
    }
Set the reference time as a string value
container-search/src/main/java/com/yahoo/prelude/Freshness.java