How to subtract time and date

Question

I have an ArrayList that has times and dates added like so

06/18/2015 12:25
06/19/2015 22:45

I want to know how can i get the exact time difference in hours between each both date. I have tried

long diff = Math.abs(d1.getTime() - d2.getTime());
long diffDays = diff / (24 * 60 * 60 * 1000);