How to know the exact time to which the database was recovered

Hello,

We use the basebackup + WAL files strategy to backup our database i.e.
take a basebackup every day and copy WAL files to a remote server every
15 minutes. In case of a disaster on the master, we'd recover the
database on the slave. If this happens, I would like to tell the
customer the exact time till when the database was recovered. How do I
get this timestamp?

Thanks.
Nikhil