JUnit assertEquals Changes String

The square brackets are emphasising the difference between the expected string and the actual string.

JUnit put the square brackets around the :00 to emphasise that that is what's in the expected string and not in the actual string. There are square brackets around the space in the actual string for the same reason.

JUnit is just putting the characters in your string that weren't equal in brackets to make it easier to read. Your assert looks for 4 sets of ":00" and your variable only had 3 sets.

As noted in this SO question (Java: Is assertEquals(String, String) reliable?), assertEquals just calls the .equals method on the objects that you pass it.