How to automatically archive a directory?

Sounds like a perfect task for rsync

sudo rsync -az /path_to/A /path_to/B

-a archive mode (implies recursive, copy symlinks as symlinks, preserve owner, modification times, group, owner, special and device files)

-z compresses the data

If you wish to remove files deleted in A from files in B, use the --delete option

For additional information see:

https://help.ubuntu.com/community/rsync

You can run rsync from cron

sudo crontab -e

Add in an hourly task

@hourly rsync /path_to/A /path_to/B

https://help.ubuntu.com/community/CronHowto

I'd suggest using rsync for this purpose. Rsync is extremely fast, stable, and versatile. There's a good introduction at http://help.ubuntu.com/community/rsync

If you wish, there is an optional graphic front end: grsync

 sudo rsync -azv --exclude 'dir1' /home/path/folderA/ /home/path/folderB

The command above will copy from folderA to folderB excluding dir1. The flags are

-a preserves time stamps
-z is to enable compression
-v verbose

There are many more options available.