To get the year, month, and day you can use the %date%
environment variable and the :~
operator. %date%
expands to something like Thu 08/12/2010 and :~
allows you to pick up specific characters out of a variable:
set year=%date:~10,4%
set month=%date:~4,2%
set day=%date:~7,2%
set filename=%year%_%month%_%day%
Use %time%
in similar fashion to get what you need from the current time.
set /?
will give you more information on using special operators with variables.