Go back in time from a PostgreSQL backup using xlog I have a PostgreSQL backup and WAL archived before the db snapshot was taken.

Is it possible to restore the database using PITR to a time before the snapshot was taken?
No. You can't.
PITR can let you go forward in time, not backward.

You have to have a backup taken before your WAL were created and you must have each WAL between your backup and the time you want to reach.

You can find this documentation helpfull.
https://www.postgresql.org/docs/current/static/continuous-archiving.html