I. Introduction
Knowledge Graphs (KGs) are used to display the complex relations between entities. As a multi-relational and directed graph, Knowledge Graph is constructed on a specific set of entities and predicates. Entities in KG can be real-world objects or abstract concepts and predicates are the possible relations between two entities. Traditional KGs do not take into account the temporal validity of facts. However, most facts only exist at a specific timestamp or time interval in the real world. Some researchers incorporate time element into KGs and extend static KGs to TKGs. Each fact in TKGs can be represented by a quadruple, (subject, action, object, time node), i.e., (s, p, o, t). Compared with SKGs, TKGs are more informative and closer to the real world, which leads to a wider range of application scenarios.