 <pre><code>/ is a culture sensitive data seperator character in your format String :`"MM/dd/yyyy"` </code></pre> <p>And the Final argument <code>null</code> means you specified to use Current Culture. Check you current culture. Is it <code>en-US</code> ? OR you can also Use <code>InvariantCulture</code></p> <p>use this:</p> <pre><code>DateTime dt =DateTime.ParseExact("19/10/2009", "MM/dd/yyyy", new CultureInfo("en-US")); </code></pre> <p>OR Use InvariantCulture</p> <pre><code>DateTime dt = DateTime.ParseExact("19/11/2011", "MM/dd/yyyy", CultureInfo.InvariantCulture); </code></pre> <p><a href="http://msdn.microsoft.com/en-us/library/w2sa9yss%28VS.80%29.aspx" rel="nofollow">Read MSDN</a> for complete details.</p>