how to update datetimepicker

I have two datetimepicker in my form

datetimepicker1 is called StartTimePicker for start time for job
datetimepicker2 is called EndTimePicker for end time for job
in properties for StartTimePicker I set Format= custom and
custom format = 08:00 MM/dd/yyyy
in properties for EndTimePicker I set Format = custom and
custom foramt = 18:00 MM/dd/yyyy
starttimepicker and Endtimepicker is inserted the current date
when I want to change date I can't do that
for example if current date is 9/29/2014
starttimepicker is inserted 9/29/2014
my problem is If I want to back date to 9/28/2014 I can't
starttimepicker is still 9/29/2014
how to solve this problem?