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The MooTools DatePicker used in the most recent versions of ChronoForms has a default date setting using the 'International' format YYYY-MM-DD; if this is OK then it is simplest not to change it. If you need another format, then please continue.
The simplest place to set a format is in the form General Tab, click the form name link in the Forms Manager to access it. Scroll down to the bottom of the page to see the DatePicker options.
If you are using a Joomla! version 1.6 or later, please make sure that you have the MooTools datepicker selected as the older MonkeyPhysics datepicker may not work correctly.
To set a Date format add an entry in the DateTime Picker config box like this: format: '%m-%d-%Y' which will give e.g. 11-16-2013
You can see the full range of options and more examples in the MooTools documents here.
You can add other options for the datepciker in the same box separated by commas. e.g. format: '%m-%d-%Y', startDate: '2013-01-10'
Note that if you are saving the data in the database and want to use a DATE or DATETIME column then you must make sure that the date is in a format that can be correctly read by MySQL before the DB Save action. The default international format is OK; the US format is not. If you set the US format in the form then you will need to add Custom Code in the On Submit event of your form to create a re-formatted date to be saved in the database table.