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.
In regression analysis, variables can be independent, which are used as the predictor or causal input and dependent, which are used as response variables. In experimental studies, independent variable X is the variable that can be controlled and variable Y is the variable that reflects the changes in the independent variable X.