Don’t Understand Date Format In Dataset

I need assistance with a dataset on sea level rise that I downloaded from CSIRO. In the “time” column, there is a record labeled “1880.9583.” Could you please clarify what the behind dot portion, “.9583,” represents in this context? A decimal portion?

http://www.cmar.csiro.au/sealevel/GMSL_SG_2011_up.html