diff --git a/acme_common/Cargo.toml b/acme_common/Cargo.toml
index 13f55a9..aa71234 100644
--- a/acme_common/Cargo.toml
+++ b/acme_common/Cargo.toml
@@ -23,6 +23,7 @@ daemonize = "0.4"
 env_logger = "0.7"
 glob = "0.3"
 handlebars = "3.0"
+libc = "0.2"
 log = "0.4"
 native-tls = "0.2"
 openssl = { version = "0.10", optional = true }
diff --git a/acme_common/src/crypto.rs b/acme_common/src/crypto.rs
index 21f3ca0..64f0fa8 100644
--- a/acme_common/src/crypto.rs
+++ b/acme_common/src/crypto.rs
@@ -10,6 +10,7 @@ mod openssl_keys;
 
 pub const TLS_LIB_NAME: &str = env!("ACMED_TLS_LIB_NAME");
 pub const TLS_LIB_VERSION: &str = env!("ACMED_TLS_LIB_VERSION");
+pub const CRT_NB_DAYS_VALIDITY: u32 = 7;
 
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum BaseHashFunction {
diff --git a/acme_common/src/crypto/openssl_certificate.rs b/acme_common/src/crypto/openssl_certificate.rs
index 5d51717..6593b9d 100644
--- a/acme_common/src/crypto/openssl_certificate.rs
+++ b/acme_common/src/crypto/openssl_certificate.rs
@@ -16,7 +16,6 @@ const APP_ORG: &str = "ACMEd";
 const APP_NAME: &str = "ACMEd";
 const X509_VERSION: i32 = 0x02;
 const CRT_SERIAL_NB_BITS: i32 = 32;
-const CRT_NB_DAYS_VALIDITY: u32 = 7;
 const INVALID_EXT_MSG: &str = "Invalid acmeIdentifier extension.";
 
 fn get_digest(digest: HashFunction, key_pair: &KeyPair) -> MessageDigest {
@@ -107,7 +106,7 @@ impl X509Certificate {
     }
 
     pub fn expires_in(&self) -> Result<Duration, Error> {
-        let timestamp = SystemTime::now().duration_since(UNIX_EPOCH)?.as_secs() as i64;
+        let timestamp = SystemTime::now().duration_since(UNIX_EPOCH)?.as_secs() as libc::time_t;
         let now = Asn1Time::from_unix(timestamp)?;
         let not_after = self.inner_cert.not_after();
         let diff = now.diff(not_after)?;
@@ -172,7 +171,7 @@ fn gen_certificate(
     builder.set_pubkey(&key_pair.inner_key)?;
     let not_before = Asn1Time::days_from_now(0)?;
     builder.set_not_before(&not_before)?;
-    let not_after = Asn1Time::days_from_now(CRT_NB_DAYS_VALIDITY)?;
+    let not_after = Asn1Time::days_from_now(super::CRT_NB_DAYS_VALIDITY)?;
     builder.set_not_after(&not_after)?;
 
     builder.append_extension(BasicConstraints::new().build()?)?;
diff --git a/acme_common/src/tests/certificate.rs b/acme_common/src/tests/certificate.rs
index d43bf5c..2feb950 100644
--- a/acme_common/src/tests/certificate.rs
+++ b/acme_common/src/tests/certificate.rs
@@ -1,4 +1,4 @@
-use crate::crypto::{HashFunction, KeyType, X509Certificate};
+use crate::crypto::{HashFunction, KeyType, X509Certificate, CRT_NB_DAYS_VALIDITY};
 use std::collections::HashSet;
 use std::iter::FromIterator;
 
@@ -52,6 +52,34 @@ eX1G4/Av0XE7RqJ3Frq8qa5EjjLJTw0iEaWS5NGtZxMqWEIetCgb0IDZNxNvbeAv
 mmH6qnF3xQPx5FkwP/Yw4d9T4KhSHNf2/tImIlbuk3SEsOglGbKNY1juor8uw+J2
 5XsUZxD5QiDbCFd3dGmH58XmkiQHXs8hhIbhu9ZLgp+fNv0enVMHTTI1gGpZ5MPm
 -----END CERTIFICATE-----"#;
+const CERTIFICATE_EXPIRED_PEM: &str = r#"-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----"#;
 
 #[test]
 fn test_san_domains() {
@@ -132,3 +160,22 @@ fn generate_ed448_certificate() {
             .unwrap();
     assert_eq!(kp.key_type, KeyType::Ed448);
 }
+
+#[test]
+fn cert_expiration_date_future() {
+    let (_, crt) =
+        X509Certificate::from_acme_ext("example.org", "", KeyType::EcdsaP256, HashFunction::Sha256)
+            .unwrap();
+    let duration = crt.expires_in().unwrap().as_secs();
+    let validity_sec = CRT_NB_DAYS_VALIDITY as u64 * 24 * 60 * 60;
+    let delta = 60;
+    assert!(duration > validity_sec - delta);
+    assert!(duration < validity_sec + delta);
+}
+
+#[test]
+fn cert_expiration_date_past() {
+    let crt = X509Certificate::from_pem(CERTIFICATE_EXPIRED_PEM.as_bytes()).unwrap();
+    let duration = crt.expires_in().unwrap().as_secs();
+    assert_eq!(duration, 0);
+}