Just noticed that you can define an interval for minutes when using the form helper (up to now I used my own little helper to accomplish this).
In the following example I use an interval of 15 minutes, i.e. the generated select box for the minutes only contains the values “00”, “15”, “30”, and “45”:
echo $form->input('start', array('type' => 'time', 'interval' => 15));
The default interval is one minute, but from the user’s point of view it is not that user-friendly to have a select box with 60 values, so I recommend to specify a different interval as shown above.