I downloaded CAMS Global atmospheric composition data. The data server web interface offers two daily timestamps, 00:00 an d12:00 UTC, but the downloaded data contains only the data for 00:00 UTC, data for 12:00 UTC is missing.
Until 21 June 2016 CAMS Global data was produced only once a day, with basetime 00:00 UTC.
From 21 June 2016 CAMS Global data is produced twice daily, with basetime 00:00 and 12:00 UTC.
The data server web interface does not reflect this change and always offers both basetimes.
For more detail on changes in the CAMS Global atmospheric composition service please see https://atmosphere.copernicus.eu/user-support/operational-info/global-system-changes