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copied!<p><a href="http://download.oracle.com/javase/6/docs/api/constant-values.html#java.util.Calendar.JANUARY" rel="nofollow noreferrer"><code>Calendar.JANUARY</code></a> is actually <code>0</code>, not <code>1</code>.</p> <p>When you provide <code>01</code> for the month field in <code>set</code>, you're actually setting the month to February, which is why you get <code>02</code> when <code>SimpleDateFormat</code> renders it as <code>MM</code>.</p> <p>When you use any of the <code>Calendar.get/set</code> methods, you must take extra precautions to make sure that you are aware of this discrepancy between the "natural" 1-based indexing, and the more "awkward" <code>Calendar</code>'s 0-based indexing. Any time you're getting/setting the month of a <code>Calendar</code>, there's always this potential to cause a serious bug.</p> <p>This is just one of those really awkward design in <a href="http://java.sun.com/javase/6/docs/api/java/util/Calendar.html" rel="nofollow noreferrer"><code>Calendar</code></a> that leaves a lot to be desired. One of the better, more pleasant date/time API library available out there is <a href="http://joda-time.sourceforge.net/" rel="nofollow noreferrer">Joda Time</a>, so if at all possible, you may consider a switch to that library instead.</p> <h3>API links</h3> <ul> <li><a href="http://download.oracle.com/javase/6/docs/api/java/util/Calendar.html#MONTH" rel="nofollow noreferrer"><code>Calendar.MONTH</code></a> - "Field number for <code>get</code> and <code>set</code> indicating the month. This is a calendar-specific value. The first month of the year [...] is <code>JANUARY</code> which is 0".</li> <li><a href="http://download.oracle.com/javase/6/docs/api/java/util/Calendar.html#set%28int,%20int,%20int,%20int,%20int%29" rel="nofollow noreferrer"><code>Calendar.set(…, int month, …)</code></a> - "<code>month</code> - the value used to set the <code>MONTH</code> calendar field. Month value is 0-based. e.g., 0 for January."</li> </ul> <hr> <h3>On octal literals</h3> <p>Also, note that <code>01</code> is actually an octal literal (i.e. <a href="http://en.wikipedia.org/wiki/Octal" rel="nofollow noreferrer">base 8</a>). You shouldn't make a habit of prepending <code>0</code> to integer literals (<a href="http://java.sun.com/docs/books/jls/third_edition/html/lexical.html#3.10.1" rel="nofollow noreferrer">§3.10.1</a>), because they can cause subtle bugs/errors unless you're very careful.</p> <p>For example, <code>int i = 09;</code> is an illegal Java code.</p> <pre><code> System.out.println(010); // prints 8, not 10 </code></pre> <h3>See also</h3> <ul> <li><a href="https://stackoverflow.com/questions/2398208/can-i-customize-syntax-highlighting-in-eclipse-to-show-octal-literals-differently">Can I customize syntax highlighting in Eclipse to show octal literals differently?</a></li> <li><a href="https://stackoverflow.com/questions/44569/octal-number-literals-when-why-ever"> Octal number literals: when? why? ever? </a></li> </ul>

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