UIDatePicker upon date changed

When properly configured, a
UIDatePicker object sends an action
message when a user finishes rotating
one of the wheels to change the date
or time; the associated control event
is UIControlEventValueChanged.

so you need to add your class to handle UIControlEventValueChanged event in your picker:

[picker addTarget:self action:@selector(dateChanged:) 
              forControlEvents:UIControlEventValueChanged];
...

- (void) dateChanged:(id)sender{
   // handle date changes
}