This is referred to in mission planning as Hybrid Local Solar Time (HLST). The time scales are uniform in the sense of mean solar time (they are actually mean time of some longitude), and are not adjusted as the rovers travel. (The rovers have traveled distances that make a few seconds difference to local solar time.)
Answer: An independent variable is exactly what it sounds like. It is a variable that stands alone and isn't changed by the other variables you are trying to measure. For example, someone's age might be an independent variable.
The independent variable is independent because its variation does not depend on the variation of another variable in the experiment/research project. The independent variable is controlled or changed only by the researcher. This factor is often the research question/hypothesis behind the outcome of the experiment. In the above example, the researcher may have wanted to see if participating in the training program raised students' scores on a final test.