the "time dependency" means that on January 15th only AAA & BBB belong to LEBENSMITTEL and CCC & DDD belong to CHEMIKALIEN
if the key date changes to February 15th, then only AAA will belong to LEBENSMITTEL & BBB, CCC, DDD & EEE will belong to CHEMIKALIEN
so your results will completely change if you report on material group
now, if you were to report on material itself, there would be no issue, meaning sales of January would be 400 & sales of February would be 500