Pre-trained langauge models have achieved success in many natural language processing tasks, whereas they are trapped by the time-agnostic setting, impacting the performance in automatic text dating. This paper introduces TicTac, a supervised fine-tuning model for automatic text dating. Unlike the existing models that always ignore the temporal relatedness of documents, TicTac has the ability to learn temporal semantic information, which is helpful for capturing the temporal implications over long-time span corpora. As a fine-tuning framework, TicTac employs a contrastive learning-based approach to model two types of temporal relations of diachronic documents. TicTac also adopts a metric learning approach, where the temporal distance between a historical text and its category label is estimated, which benefits to learn temporal semantic information on texts with temporal ordering. Experiments on two diachronic corpora show that our model effectively captures the temporal semantic information and outperforms state-of-the-art baselines.