Unclear if timestamps in text format must be milliseconds or seconds

[aspyct]

Hello,

The exposition format documentation specifies that the timestamps must be expressed in milliseconds:

The timestamp is an int64 (milliseconds since epoch, i.e. 1970-01-01 00:00:00 UTC, excluding leap seconds), represented as required by Go's [ParseInt()](https://golang.org/pkg/strconv/#ParseInt) function.

However, when I use promtool to generate blocks from data, it seems to expect timestamps in seconds. Also, the OpenMetrics specifications (which I believe is the exposition format explained by the other document) says that the timestamp must be specified in seconds:

Timestamps MUST be Unix Epoch in seconds.

Are these two different formats/usages?

Related to another ticket, it would be nice if it were specified that prometheus cannot ingest data older than ~1h.

A link to the promtool backfilling tools would be nice too, since that's probably what people are looking for.