Written
ChronoForms v4 has two different date-pickers included. The default is the MonkeyPhysics datepicker, the alternative is the MooTools date-picker. Unless you are still using Joomla! 1.5 you should always use the MooTools datepicker.
The MonkeyPhysics Datepicker has not been updated to work with the current versions of the MooTools library and can be buggy when used on Joomla! 1.6 or later.
The setting to change the date-picker is on the form General tab, click the form name link in the Forms Manager to open it and scroll down to the DatePicker settings.