Stochastic temporal reasoning in situation assessment (SA) is very important in many applications. The approach based on Maximun Likelihood Estimation (MLE) treats the unknown temporal variable as a constant, which doesn't use a priori information and generates a larger/estimate variance. In this paper, the relation model of known temporal information and unknown temporal variable has been established, which can also be used for MLE-based method. In the model, the reasoning algorithm about time instants has been derived from treating the unknown temporal variable as random variable and introducing MAP estimation into temporal reasoning. The performance analysis between MAP-based method and MLE-based method shows that under some conditions, the estimate variance of MAP-based method is lower than that of the MLE-based method, and we have given these conditions in experiments.