A digital signature will fail to verify if any change has occurred to the data between the time it was originally signed and the time the signature is verified. This conflicts with our desire to have the time-to-live field tick down when resource records are cached.
This could be avoided by leaving the time-to-live out of the digital signature, but that would allow unscrupulous servers to set arbitrarily long time to live values undetected. Instead, we include the "original" time-to-live in the signature and communicate that data in addition to the current time-to-live. Unscrupulous servers under this scheme can manipulate the time to live but a security aware resolver will bound the TTL value it uses at the original signed value. Separately, signatures include a time signed and an expiration time. A resolver that knows the absolute time can determine securely whether a signature has expired. It is not possible to rely solely on the signature expiration as a substitute for the TTL, however, since the TTL is primarily a database consistency mechanism and, in any case, non-security aware servers that depend on TTL must still be supported.