ta_parse() is always called after cert_parse_pre() which already stores
notbefore and notafter in struct cert, so we can use a time_t comparison.
Of course this assumes that we're not on a 32-time_t operating system,
which we do elsewhere already.

Index: cert.c
===================================================================
RCS file: /cvs/src/usr.sbin/rpki-client/cert.c,v
diff -u -p -r1.136 cert.c
--- cert.c	4 Jun 2024 14:10:53 -0000	1.136
+++ cert.c	5 Jun 2024 11:29:35 -0000
@@ -1073,7 +1073,6 @@ struct cert *
 ta_parse(const char *fn, struct cert *p, const unsigned char *pkey,
     size_t pkeysz)
 {
-	ASN1_TIME	*notBefore, *notAfter;
 	EVP_PKEY	*pk, *opk;
 	time_t		 now = get_current_time();
 
@@ -1095,20 +1094,11 @@ ta_parse(const char *fn, struct cert *p,
 		    "pubkey does not match TAL pubkey", fn);
 		goto badcert;
 	}
-
-	if ((notBefore = X509_get_notBefore(p->x509)) == NULL) {
-		warnx("%s: certificate has invalid notBefore", fn);
-		goto badcert;
-	}
-	if ((notAfter = X509_get_notAfter(p->x509)) == NULL) {
-		warnx("%s: certificate has invalid notAfter", fn);
-		goto badcert;
-	}
-	if (X509_cmp_time(notBefore, &now) != -1) {
+	if (p->notbefore >= now) {
 		warnx("%s: certificate not yet valid", fn);
 		goto badcert;
 	}
-	if (X509_cmp_time(notAfter, &now) != 1) {
+	if (p->notafter <= now) {
 		warnx("%s: certificate has expired", fn);
 		goto badcert;
 	}
@@ -1139,7 +1129,7 @@ ta_parse(const char *fn, struct cert *p,
 	EVP_PKEY_free(pk);
 	return p;
 
-badcert:
+ badcert:
 	EVP_PKEY_free(pk);
 	cert_free(p);
 	return NULL;