update config file

config/common.conf

@@ -14,6 +14,11 @@ DIGESTS=""
 # default installation.
 DISTDIR="/usr/portage/distfiles"
 
+# Using this, the default tarball and image date format can be changed according to the
+# requirement. This customizable format may be needed when you want to set different
+# timestamp in output files. This option can be overridden by using -D option.
+TARBALLDATE="%Y%m"
+
 # A time designation representing the time in seconds that you allow the connection to
 # the server to take. This only limits the connection phase, once it has connected this
 # option is of no more use. Setting a timeout to 0 disables it altogether. Unless you