From one of our team meetings:
Context was, as best as I can remember: we were looking at data sources/displays for our KPIs. Since the CI data (??) is sampled after 30 days it is not displaying correctly.
I evoked that when building a Grafana dashboard for some Gerrit metrics reported by Zuul: https://grafana.wikimedia.org/dashboard/db/releng-gerrit
Some of the metrics (like number of comments) were/are aggregated by average when being a count the aggregation function should be a sum. That showed up for time period larger than 30 days. I haven't verified whether that is still the case.