Time is a flat circle means that in order to understand the present, you must look into the past. While literally any point in time can be seen as always being “now”, there are points of time which exhibit more relevance than others, depending on their distance from the present.
The phrase “time is a flat circle” was coined by author and philosopher Thomas Ligotti. It is based on the premise that history and significance emerge not from events themselves but through how we narrate them before moving to other topics. In this sense, rather than travelling endlessly forward in time, we are perpetually circling back due to our continual revisiting of past times.