JHTML/calendar
From Joomla! Documentation
Displays a calendar control field.
calendar (string $value, string $name, string $id, [string $format = '%Y-%m-%d'], [array $attribs = null])
* string $value: The date value * string $name: The name of the text field * string $id: The id of the text field * string $format: The date format * array $attribs: Additional html attributes