Every market payload carries a time. Almost none of them are clear about what that time refers to, and the ambiguity is not harmless — it is the difference between a reading taken thirty seconds ago and one taken at yesterday’s close, wrapped in a response generated just now.
The three meanings
A timestamp can mean measured: we computed this value at this moment. It can mean retrieved: somebody else published it, and this is when we fetched it. Or it can mean unknown: we hold the value, and we genuinely cannot say when it was true.
Collapsing those three into one field is how a feed ends up stamping the moment it built the response onto a number that came from a weekly government release. The response is new. The fact is not.
What to ask a feed for
Three fields, not one. When it was as of, how many seconds old that makes it, and which of the three meanings applies. With those, a consumer can decide for itself whether the number is usable for what it is about to do.
Without them you are left inferring freshness from response time, which measures our servers rather than the market.
Absent is not empty
The related failure is quieter. When an upstream source fails, the tempting thing is to return an empty list with a current timestamp — which reads to any consumer as “we looked just now and there is nothing there.” That is a different claim from “we could not look,” and rendering the second as the first turns an outage into a calm, wrong market report.
A feed worth trusting distinguishes them: a named reason when it could not read, and no invented time on a result it did not measure.