Some ECMWF parameters are described as being valid at the specified time. Sometimes they are referred to as "instantaneous" parameters, and may even have "instantaneous" in their name. Parameters such as these have a designated validity time eg 12:00 UTC. However, they are not really instantaneous because they represent an average over the model time step. Some forecast and most, if not all, analysed parameters fall into this category.
Note, that in addition, such parameters may, or may not, be averaged in time. For example, they might be averaged over a month from hourly data, in which case they would be archived in a monthly mean "stream". This would be indicated in other metadata in the GRIB header of the data, as would details of the averaging frequency and period.
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