Since leap second is possible only at the end of month. So the most recent leap second are in the following list (ftp://tycho.usno.navy.mil/pub/series/ser14.txt):
0 36204 01-01-58 Beginning of Atomic Time
 1 41499 07-01-72 Effective at 0 hours UTC of the MJD
 2 41683 01-01-73
 3 42048 01-01-74
 4 42413 01-01-75
 5 42778 01-01-76
 6 43144 01-01-77
 7 43509 01-01-78
 8 43874 01-01-79
 9 44239 01-01-80
10 44786 07-01-81
11 45151 07-01-82
12 45516 07-01-83
13 46247 07-01-85
14 47161 01-01-88
15 47892 01-01-90
16 48257 01-01-91
17 48804 07-01-92
18 49169 07-01-93
19 49534 07-01-94
20 50083 01-01-96
21 50630 07-01-97
22 51179 01-01-99

Therefore the time to have leap second is only 0 hours and 0 minutes (or 23 hours and 59 minutes). Other times may not have leap second and following test should throw IllegalArgumentException:

import javax.xml.datatype.DatatypeConfigurationException;
import javax.xml.datatype.DatatypeFactory;
import javax.xml.datatype.XMLGregorianCalendar;


class test {

    public static void main(String [] args) throws Exception {
        DatatypeFactory factory = null;
        try {
            factory = DatatypeFactory.newInstance();
        } catch (DatatypeConfigurationException dce) {
            dce.printStackTrace();
        }

        XMLGregorianCalendar calendar =
                factory.newXMLGregorianCalendar();
        calendar.setTime(3, 23, 60);
        System.out.println("OK");
    }
}

But it prints OK.

###@###.### 2005-03-10 15:36:24 GMT

intrgrated in tag JWSDP-1_6-b08
###@###.### 2005-05-02 23:05:20 GMT