String was not recognized as a valid DateTime " format dd/MM/yyyy"

Question

Use DateTime.ParseExact.

this.Text="22/11/2009";

DateTime date = DateTime.ParseExact(this.Text, "dd/MM/yyyy", null);

You need to call ParseExact, which parses a date that exactly matches a format that you supply.

For example:

DateTime date = DateTime.ParseExact(this.Text, "dd/MM/yyyy", CultureInfo.InvariantCulture);

The IFormatProvider parameter specifies the culture to use to parse the date.
Unless your string comes from the user, you should pass CultureInfo.InvariantCulture.
If the string does come from the user, you should pass CultureInfo.CurrentCulture, which will use the settings that the user specified in Regional Options in Control Panel.

Parsing a string representation of a DateTime is a tricky thing because different cultures have different date formats. .Net is aware of these date formats and pulls them from your current culture (System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat) when you call DateTime.Parse(this.Text);

For example, the string "22/11/2009" does not match the ShortDatePattern for the United States (en-US) but it does match for France (fr-FR).

Now, you can either call DateTime.ParseExact and pass in the exact format string that you're expecting, or you can pass in an appropriate culture to DateTime.Parse to parse the date.

For example, this will parse your date correctly:

DateTime.Parse( "22/11/2009", CultureInfo.CreateSpecificCulture("fr-FR") );

Of course, you shouldn't just randomly pick France, but something appropriate to your needs.

What you need to figure out is what System.Threading.Thread.CurrentThread.CurrentCulture is set to, and if/why it differs from what you expect.

Although the above solutions are effective, you can also modify the webconfig file with the following...

<configuration>
   <system.web>
     <globalization culture="en-GB"/>
   </system.web>
</configuration>

Ref : Datetime format different on local machine compared to production machine

You might need to specify the culture for that specific date format as in:

    Thread.CurrentThread.CurrentCulture = new CultureInfo("en-GB"); //dd/MM/yyyy

    this.Text="22/11/2009";

    DateTime date = DateTime.Parse(this.Text);

For more details go here:

http://msdn.microsoft.com/en-us/library/5hh873ya.aspx