Just looking at your screenshot the timestamp format looks like Unix Time http://en.wikipedia.org/wiki/Unix_Time
but with 3 extra digits (so expressed in milliseconds rather than seconds).
Since added_date is higher than modified_date I'd keep reading_time greater than added_date. Also going on my hunch and judging from added time a unix time converter gives me an estimated 'time added to reader' of: 07 / 06 / 12 @ 1:07:50am EST (for the first book in your screenshot)
If that seems pretty much right then my hunch about the timestamp format is probably right. And as I'm considering doing a full reload myself (sorting out my collections slightly differently) I'll be watching this thread with interest.