Measurin the lag between a master and a replica

Hi,
I am using a master and a replica with PostgreSQL 9.1.3.
I would like to measure the replication lag between my servers (in time units).

On the replica I can use the function pg_last_xact_replay_timestamp().
But on the master I only can use pg_current_xlog_location() which does not return a timestamp. I do not know any function returning the timestamp.

Is there any way to get the timestamp relative to the result of pg_current_xlog_location() ?

Thanks for your help
Best regards
Ali Pouya