FULL PRODUCT VERSION :
java version " 1.6.0_45 "
Java(TM) SE Runtime Environment (build 1.6.0_45-b06)
Java HotSpot(TM) 64-Bit Server VM (build 20.45-b01, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

Function is used within JEE environment. switch Java 7 is excluded at the time!

EXTRA RELEVANT SYSTEM CONFIGURATION :
None Eclipse Juno IDE

A DESCRIPTION OF THE PROBLEM :
Simple Program:
private final static String DATE_FORMAT_STR = " yyyy-MM-DD HH:mm:ss " ;

private final static String DATE_STR_1 = " 2012-12-17 14:29:15 " ;
private final static String DATE_STR_2 = " 2013-08-23 11:53:18 " ;

public static void main(String[] args) throws ParseException {
DateFormat df = new SimpleDateFormat(DATE_FORMAT_STR);
Date myDate = df.parse(DATE_STR_1);
System.out.println( " date 1: " + myDate);
Date myDate2 = df.parse(DATE_STR_2);
System.out.println( " date 2: " + myDate2);
Output:
date 1:Tue Jan 17 14:29:15 CET 2012
date 2:Wed Jan 23 11:53:18 CET 2013
For DATE_STR_1 rather Dec is expected.
For DATE_STR_2 rather Aug is expected.

But both has Jan as output!

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
run the programm snippet

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
For DATE_STR_1 rather Dec is expected.
For DATE_STR_2 rather Aug is expected.
ACTUAL -
date 1:Tue Jan 17 14:29:15 CET 2012
date 2:Wed Jan 23 11:53:18 CET 2013

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
private final static String DATE_FORMAT_STR = " yyyy-MM-DD HH:mm:ss " ;

private final static String DATE_STR_1 = " 2012-12-17 14:29:15 " ;
private final static String DATE_STR_2 = " 2013-08-23 11:53:18 " ;

public static void main(String[] args) throws ParseException {
DateFormat df = new SimpleDateFormat(DATE_FORMAT_STR);
Date myDate = df.parse(DATE_STR_1);
System.out.println( " date 1: " + myDate);
Date myDate2 = df.parse(DATE_STR_2);
System.out.println( " date 2: " + myDate2);
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
None actually