diff --git a/src/acme.c b/src/acme.c
index e71071692..b5746a41a 100644
--- a/src/acme.c
+++ b/src/acme.c
@@ -2381,7 +2381,7 @@ int acme_will_expire(struct ckch_store *store)
  */
 time_t acme_schedule_date(struct ckch_store *store)
 {
-	int diff = 0;
+	time_t diff = 0;
 	time_t notAfter = 0;
 	time_t notBefore = 0;
 
@@ -2395,7 +2395,8 @@ time_t acme_schedule_date(struct ckch_store *store)
 	notAfter = x509_get_notafter_time_t(store->data->cert);
 	notBefore = x509_get_notbefore_time_t(store->data->cert);
 
-	if (notAfter >= 0 && notBefore >= 0) {
+	if ((notAfter >= 0 && notBefore >= 0)
+	   && (notAfter > notBefore)) {
 		diff = (notAfter - notBefore) / 12; /* validity period / 12 */
 	} else {
 		diff = 7 * 24 * 60 * 60; /* default to 7 days */