Testing the basics: 

d1 = 5 hours, 10 minutes, and 0 seconds
d2 = 1 hour, 2 minutes, and 3 seconds
d3 = 1 hour, 2 minutes, and 3 seconds
d3 = 5 hours, 10 minutes, and 0 seconds
d4 = 0 hours, 0 minutes, and 0 seconds

Testing arithmetic: 

d1 = 106 hours, 46 minutes, and 45 seconds
d2 = 0 hours, 0 minutes, and 0 seconds
d3 = 101 hours, 41 minutes, and 40 seconds
d4 = 49 hours, 50 minutes, and 51 seconds
d2 / d3 = 0.5
d3 / d2 = 2

Testing unusual cases: 

d1 = 1 hour, 0 minutes, and 1 second
d1 = 99 hours, 59 minutes, and 59 seconds
d2 = 0 hours, 0 minutes, and 5 seconds
d3 = 1 hour, 0 minutes, and 0 seconds

Test the randomize function: 

d1 = 21 hours, 44 minutes, and 55 seconds
d1 = 2 hours, 3 minutes, and 50 seconds
d1 = 5 hours, 53 minutes, and 22 seconds
d1 = 15 hours, 8 minutes, and 26 seconds
d1 = 4 hours, 6 minutes, and 12 seconds
d1 = 8 hours, 41 minutes, and 17 seconds
d1 = 21 hours, 53 minutes, and 41 seconds
d1 = 12 hours, 35 minutes, and 20 seconds
d1 = 22 hours, 21 minutes, and 47 seconds
d1 = 1 hour, 30 minutes, and 12 seconds
Average of 10,000 random durations is: 11 hours, 54 minutes, and 48 seconds