How to aggregate daily data into MODIS-like 8 day aggregation pattern?

Several MODIS products come in “8-day” compositions. Suppose we have daily data and we want to aggregate it with this MODIS-like granularity. How can we achieve that? First idea would be to use the great t.rast.aggregate module in GRASS GIS with granularity=“8 days”. But this presents a problem. It does not re-start every year, but aggregates the whole series with 8-day granularity, not considering the year. Meanwhile, MODIS 8-day products re-start the aggregation every January 1st.