problem running crontab jobs

[Luca Ferrari]

Hi all,
my crontab contains a rude way to backup my eclipse source tree:
$ crontab -l

BACKUP_DIR=/mnt/disco4/backup
DATE=`/bin/date +'%d_%m_%y'`


################### ECLIPSE #################################
00 12  * * * /bin/tar cjvf 
${BACKUP_DIR}/sorgenti_java_giorno.tar.bz2 /mnt/disco1/src/java
00 17  * * * /bin/tar cjvf 
${BACKUP_DIR}/sorgenti_java_sera.tar.bz2  /mnt/disco1/src/java
50 17  * * 1,3,5 /bin/tar cjvf 
${BACKUP_DIR}/sorgenti_java_${DATE}.bz2  /mnt/disco1/src/java

Now, the first two scripts are working, while the third (the one with the 
date) is not producing the output. If I run the same line manually, after 
having exported the BACKUP_DIR and DATE variables, it run correctly. Any idea 
about that?

Thanks,
Luca