how to disable a time passing in DtPicker with format of HH:mm

i have 2 date pickers with format of hours
now i also have a list that you can only make a appointment from 08:00 to 23:00
first dtpicker is called "DtStartTime" 08:00
second dtpicker is called "DtEndTime"23:00

how Do i disable a user from making a appointment before 08:00- cannot choose before 08:00
and finishing a appointment after 23:00 -cannot choose after 23:00
also if i choose in DtStartTime 08:00 then the DtEndTime will be 08:15
15 minutes between DtStartTime & DtEndTime

any help will be appreciated
regards salsa31