When a user launches Firefox for the first time in a few days or weeks, they can see sponsored content (spocs) for campaigns that ended long ago because the Pocket recommendations feed response hasn't had a chance to refresh before the first tab is rendered. To prevent this, Pocket will include an `expiration_timestamp` with each spoc in the /v3/firefox/global-recs response, and would need that timestamp checked as part of the filtration logic when deciding whether a spoc is eligible to be shown.

Is it correct to assume that `expiration_timestamp` is the number of seconds elapsed since January 1, 1970 00:00:00 UTC?

Confirmed with kaiser on Slack.

Commit pushed to master at https://github.com/mozilla/activity-stream https://github.com/mozilla/activity-stream/commit/71c92b34b0e51dc147a7fa653880a2ee2298e7d9 Fix Bug 1476458 - Prevent expired sponsored stories from showing to users (#4549)