I already clarified that the time capture resolution is more than enough for a minute and upper boundary of the  duration is safe for 3600 min (1d).

So, I used the integer because of several facts.
1. Since, we don't capture seconds it's waste(processing, storage and data transfer) to store data on that. Initially I used LocalTime(in Java) and Time(Mysql) for duration. When using SMALLINT instead of TIME it improves performance by around(33%) at the mysql database.
2. Data manipulation over integer is faster than using TIME format in java.
   Ex:- when using validations it uses lower overhead to data conversion and processing.  
3. It reduces the json request data packet size.

>>can you rename the column to event_start_time instead of start_time. ...
It's ok I can do that. :) ( But still it also increase the API message size. I tried to optimize as much as possible )

Thanks,
~ Diunuge