If you've done any programming, you could hook up a script to fdm (https://github.com/nicm/fdm).
Rough logic, for each message:
* match body with several timestamp regexps
* parse matched messages
* find dates in message body
* parse final match
* discard message if that is date earlier than now - x days
Might be closer than you think. The White House is just using Instagram right now: https://www.whitehouse.gov
(See section “featured media”)
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