I do not want to convert time but only a difference.
I'm sorry, but that does not make sense.
You can calculate the time since midnight, for any time, if the date doesn't matter. Do that for each time, and you simply subtract one value from the other to get the difference.
unsigned long secondsSinceMidnight = (rtc.now().hour() * 24UL * 60 * 60) +
(rtc.now().minute() * 60) +
Now, if you want the interval in hours, minutes, and seconds, instead of just seconds, that is relatively easy to do.
By the way, I do NOT recommend getting the seconds since midnight the way I showed, because you do NOT want to call now() three times. You might NOT get the same time all three times.
Store the value returned by rtc.now() in the proper type variable, and get hour(), minute(), and second() from that variable.