grdvr:
In NVR Client we cannot see which days have recorded video, as every day in the calendar is marked as blue. Is this a bug?

For your question, in local playback of NVR Client software, the days marked with gray do not have recorded video, the days marked with blue have recorded video.
Also in remote playback of NVR Client software, all the days later than the current data will be marked with gray, all the days earlier than the current date will be marked with blue, so gray and blue do not mean there are recorded video or not, of course gray days are impossible to have recorded video, but for blue days, you have to check by clicking on the date to see if there is recorded video.

Thank you for your support.