Last modification time of a database?

So, I've got loads of databases and I'd really like some way to see
what the last actual modification time was for them.  I can't just
check the time stamp on the file in the database directory as
autovacuum fudges that and trying to match up values for either
last_autovacuum or last_autoanalyze in pg_stat_all_tables/indexes with
the files' time stamps is no good as the time stamp on a file that was
last written to by autovacuum is for when autovacuum stopped writing
to it whereas the values in pg_stat_all_tables/indexes are from when
autovacuum started the given operation and the difference between the
two can vary with the size of the table.

Am I missing something obvious here?  If not, has anyone come up with
a reliable way to do this?

Erik Jones, Database Administrator
Engine Yard
Support, Scalability, Reliability
866.518.9273 x 260
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