From maybe a "logical" approach I should set the timer to "00:00:00". But I could not find a real practical use. The clear button is mainly meant for removing an item (empty text = removal of item at save). The time choosen is not relevant in that case. And even if I only wanted to edit an item by clearing the text first, I am not really helped by a reset to "00:00:00" that much, as I may want to set a time close to the old one.

@foobar:
I have to agree with mr_pingu and sixwheeledbeast. I have never made a meal that took more than 4 hours. Exending it to 99 hours is a small technical change, but it would make the timepicker more cramped. If some event really takes that long, you could perhaps better schedule an alarm using the (replacement) clock.
But you can always try to convince me with a practical example as why you would need that amount of hours (and come up with a better example then mr_pingu ).