hello everyone

i have a module name hospitalmanagement .

in this module i can create appintment with doctor .then i set two field time_from and time_to.

now i add onchange on time_from when i add time in time_from then automatic add 30 min after time in time_to (ex.. i add in time_from = 10:00 so in time_to = 10:30 automatic) 

but if i manually change in time_to so this is chnage (ex.. if i change time_to = 11:00) this is save 

now i want if anyone change time and add more than 30 min so raise error if under 30 min then save .

how can i do this any one know about that??

thank you in advance