Discovering time of last database write

Hi,

I need to be able to determine the last time (and date) that a database was written to.  I know it could be possible just to check the last modified dates in the PGDATA directory, but i need to compare the last write time of 3 databases (connecting via JDBC).  Hopefully the last write date is contained somewhere in a system table (information schema) but i have no idea of the table(s) i would need to query.

Thanks in advance,

Andy