carl, reading a time backwards (right to left) does not need to result in a valid time. For such cases, you discard the colon to assess the numbers as an equation as Christian explained.
And, to clarify my comment regarding the Scratch Pad, the test cases need to be present in the Scratch Pad in the same order, due to the reliance upon seeding the random-number generation. I would recommend copying in all three in order before testing your solution.
@Dong-hyun: there was a problem with the test suite hiding code from the solve page. There was setup code that was being suppressed from view, but was being run before each of the tests. Accordingly, the problem was still solvable, but could not be tested via the Scratch Pad.
I have copied the necessary setup code into each test case, so it should work for you now.
Note that the "rng default" line is needed for the first test case (as is included there) but not for the other two test cases (and is, therefore, not included there).