Background timings from r2d2 to actual run env seem odd

[rtodling]

I am looking more closely at the background fetching process in swell ... I am setting up a 3dfgat_atmos case and looking at the GetBackground logs this is what I see:

...R2D2DataStore/Local/geos/fc/x0048/91/2021-12-11/geos.x0048.fc.PT6H.2021-12-11T15:00:00Z.bkg.nc to /discover/nobackup/rtodling/SwellExperiments/swell-3dfgat_atmos/run/20211212T000000Z/geos_atmosphere/bkg.20211211T210000Z.nc4

When I read this I see a 15Z file being pulled and named as a 21Z file ...

I can't make send of this ...

I simply cannot understand why files that belong to different dates fall under a directory name 2021-12-11 .. even if this is the date of the starting point of a forecast it would seem not to make sense!