https://www.rfc-editor.org/rfc/rfc6376.html

x= Signature Expiration (plain-text unsigned decimal integer;
RECOMMENDED, default is no expiration). The format is the same as
in the "t=" tag, represented as an absolute date, not as a time
delta from the signing timestamp. The value is expressed as an
unsigned integer in decimal ASCII, with the same constraints on
the value in the "t=" tag. Signatures MAY be considered invalid
if the verification time at the Verifier is past the expiration
date. The verification time should be the time that the message
was first received at the administrative domain of the Verifier if
that time is reliably available; otherwise, the current time
should be used. The value of the "x=" tag MUST be greater than
the value of the "t=" tag if both are present.

This should probably use Delivery-date header first and fall back to current time ?