So, I am linux environment.
Look how I did.

[flavio@xflavio ~]$ export TAG_0=BACKUP_FULL_ORCL_$(date +%d_%m_%y)
[flavio@xflavio ~]$ rman target /
RMAN> backup as compressed backupset database tag '$TAG_0';

But there was an error

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03009: failure of backup command on ORA_DISK_1 channel at 04/30/2007 18:13:51
ORA-01264: Unable to create backup piece file name
ORA-07217: sltln: environment variable cannot be evaluated.

Did I do anything wrong?