dropdown selection

Hi,
ihave application with dropdowns and textboxes within dropdown if user selects traveltime  and enter values i should show in traveltime textbox.Here i have problem first time user enter  startime and endtime  it gets that value in travehrs textbox ,if user changes starttime or endtims  then traveltmetexbox calculates addition of previous value.
below is my code.please check if any mistakes please let me know.
dropdown1 starttim     endtime       ServiceHrs
//firsttime user enters values in all fileds
traveltime    9:00am    12:00pm      03:00:00
//if user wants to chage 9:00am to 11:am
traveltime    11:00am    12:00pm      04:00:00//but output 01:00:00
                &nb ...