diff --git a/src/certificate.rs b/src/certificate.rs
index 21e335e..0f0c9c9 100644
--- a/src/certificate.rs
+++ b/src/certificate.rs
@@ -365,6 +367,7 @@ impl Certificate {
         yubikey: &mut YubiKey,
         key: SlotId,
         serial: impl Into<Serial>,
+        not_before: Option<DateTime<Utc>>,
         not_after: Option<DateTime<Utc>>,
         subject: &[RelativeDistinguishedName<'_>],
         subject_pki: PublicKeyInfo,
@@ -385,7 +388,10 @@ impl Certificate {
                 &signature_algorithm,
                 // Issuer and subject are the same in self-signed certificates.
                 subject,
-                Utc::now(),
+                match not_before {
+                    None => Utc::now(),
+                    Some(d) => d,
+                },
                 not_after,
                 subject,
                 &subject_pki,