Confusion about datetime format

[Lankadinee]

In the DMV dataset in https://www.dropbox.com/s/5bmvc1si5hysapf/data.tar.gz?dl=0, the workload query_dmv11.json has dates such as 20066461.49726915 under the Reg_Valid_Date column. However in the CSV files we have dates in the usual format such as 20180609. Is there any reason to have a format like 20066461.49726915 in the workload? If so how to decode that? Thank you.