FORMAT_DATE_IN etc has to do with proper absolute time records. Your decimal years are, I believe, a bit fake since years have different lengths so 2020.xxxxxx is not necessarily the same date as 2021.xxxxxx. You should just treat your decimal years as Cartesian coordinates.
If you want real, fancy abs time annotations (month names, etc) you will have to plot the baseframe separately with basemap and give actual ISO timestamp start/stop etc (e.t., 2020-01-1T/2020-09-15T14:35), then overlay your Cartesian data with its own near-matching -R and same linear scaling - you probably cannot tell the difference.