[RESOLVED] 15 minutes diffrens in 2 combobox

i have 2 comboboxes
now is there a way to make them not show the same hour?

e.x
cmbstart 8:15 cmbend 8:30

if i pick 8:15 in cmbstart the cmbend needs to be 8:30

if i pick cmbend 8:30 then cmbtart is 8:15

i want to make a diffrence of 15 minutes between this 2 comoboxes

and they cannot show the same time

tnx in advanced
salsa31