I think they are the same thing.
You probably saw the word 'default' used in 'Versions' and the word 'Built in' used in 'Time'.
They are found in https://help.anaplan.com/anapedia/Content/Modeling/Dimensions/Time.html and https://help.anaplan.com/anapedia/Content/Modeling/Dimensions/Versions.html
Both are standard to Anaplan, in the sense that they will always exist with every new model you create. And you won't be able to delete them.
Thanks a lot.0