Interesting problem - not really one considered in the original design.
I did muse here (Dashboard - Loggedin vs Public - #37 by borpin) if each Feed should have a timezone attribute to try and provide a route to solving some of the TZ issues.
It would work retrospectively (as UNIX time is used internally) and could default to the TZ of the user creating the Feed.
@TrystanLea, @glyn.hudson @Gwil, thoughts around the practicality of that?