1. Introduction  
2. Fundamentals of time synchronization in communication networks  
3. Basic models and techniques of time keeping and time synchronization  
3.2. Software clocks  
3.4. Clock offset, skew and drift  
3.5. Main limiting factors in time synchronization  
3.7. Types of clock synchronization  
4. Approaches to the time synchronization in wireless media  
4.1. Revisiting time synchronization in wireless sensor networks  
4.2. Why is time synchronization an issue in VANETs  
4.3. Requirements analysis for time synchronization in VANET  
5. Advances towards VANET time synchronization  
5.1. Communication model  
6. GNSS approaches for VANET time synchronization  
6.1. Motivation of GNSS-driven time synchronization in VANETs  
6.2. GNSS time synchronization models for VANET  
6.3. Challenges and solutions in the absence of GNSS signals  
7. Concluding remarks