We consider time-dependent systems and their possible states or situations. In order to do this, we define suitable algebraic structures, including time. Situation descriptions are represented with the corresponding formulas and we look at inferences from formulas, where the inference may be partially incorrect. We define the measure of correctness and show how it can be used to compare the plausibility of the described situations. Keywords. Time-dependent systems. Developments and deduction. Measure of correctness of inference. Plausibility.