Four dates that should never be confused
Learn the difference between an observation date, source publication date, source update date and retrieval time in outbreak reporting.
Observation: when the event happened
A case may be assigned to the date symptoms began, the date a sample was taken or the date it was reported. These are different clocks. A chart should state which date it uses. Comparisons can be misleading if one series uses symptom onset and another uses notification.
Publication: when the source reported it
The original publication date belongs to the source report. It should not be replaced by the date an aggregator imported a link. If an article is revised, the revision needs its own description and date.
Retrieval: when a system checked
A successful source check shows that a request or validation succeeded. It does not mean that the source issued a new report. An HTTP not-modified response should preserve the existing content and its dates.
Local changes: what this site edited
A substantive editorial change can justify a local modification date. A cache refresh, a routine poll or a new advertising impression cannot. When time zones matter, use an explicit zone and retain enough information to reconstruct the original time.
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