GRIB time stamps of ERA5 Monthly single levels download


I want to download some variable from the dataset: ERA5 monthly averaged data on single levels from 1940 to present. I downloaded it in GRIB format. 

When I inspect the GRIB file using the python package osgeo, the GRIB_REF_TIME and the GRIB_VALID_TIME both give the same slightly off looking time stamp.

To figure out the problem I downloaded two months: January 2021 and Feb 2021. 

Respective time stamps were:

Date EPOCH time stamp Converted to date
Jan 2021 1609437600 2020-12-31 18:00:00
Feb 2021 1612116000 2021-01-31 18:00:00

Does this just mean that the timestamps somehow have shifted 6 hours back in time and I can fix it by adding 6 hours back to it. Or does the 2020-12-31 timestamp correspond to December? I assume the frist one, but I cant find confirmation in the docs. 

When I downloaded data from the Essential Climate Variables, the time stamps corresponded to the first day of the month 00:00. 

Thanks in advance!

Also I'm not a very experienced CDS user and it's my first time using the forum, sorry if question is too simple. 

Code I used:

from osgeo import gdal

grib_file = "ERA5_mo_avg_single_levels_UV_jan_feb_2021.grib"
grib = gdal.Open(grib_file)
band1 = grib.GetRasterBand(1)
band2 = grib.GetRasterBand(2)