Descubierto de tiempo en HTML
for($hours=0; $hours<24; $hours++) // the interval for hours is '1'
for($mins=0; $mins<60; $mins+=30) // the interval for mins is '30'
echo '<option>'.str_pad($hours,2,'0',STR_PAD_LEFT).':'
.str_pad($mins,2,'0',STR_PAD_LEFT).'</option>';
Ankur