Google Calendar is a popular online calendar tool offered by Google. There are several synonyms for Google Calendar that can be used depending on the context and preference. One of the most common synonyms is "online calendar," which refers to any digital calendar that can be accessed online. Another synonym for Google Calendar is "web calendar," which emphasizes the fact that it can be accessed through a web browser. Alternatively, "cloud calendar" can also be used since Google Calendar stores data in the cloud. Lastly, "digital planner" and "schedule tool" are also good synonyms for Google Calendar since it helps users manage their time and schedule.