Apache Reference: mod_expires, ExpiresDefault
Default Value for Generation of Expires Header
Syntax: ExpiresDefault codeA
Example: ExpiresDefault M86400
Since: Apache 1.2
This directive sets the default for calculating the expiration
time for all documents in the affected realm. It can be overridden
on a type-by-type basis by the ExpiresByType directive. The
syntax of the codeseconds argument is the same as that for
ExpiresByType.