Update: Now that it\'s 2016 I\'d use PowerShell for this unless there\'s a really compelling backwards-compatible reason for it, particularly because of the regional setting
This is what I've used:
::Date Variables - replace characters that are not legal as part of filesystem file names (to produce name like "backup_04.15.08.7z")
SET DT=%date%
SET DT=%DT:/=.%
SET DT=%DT:-=.%
If you want further ideas for automating backups to 7-Zip archives, I have a free/open project you can use or review for ideas: http://wittman.org/ziparcy/