diff --git a/configure b/configure
index 641657b12..2b4149c0c 100755
--- a/configure
+++ b/configure
@@ -4462,11 +4462,6 @@ $as_echo "no" >&6; }
 
 fi
 
-# ommit -O2 for clang because of optimizer failures (in expi-incep in
-# the signature serial timestamp checks.)
-if $CC --version 2>&1 | grep "clang" >/dev/null; then
-	:
-else
 
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC supports -O2" >&5
@@ -4500,7 +4495,6 @@ fi
 
 default_cflags=yes
 fi
-fi
 ac_ext=c
 ac_cpp='$CPP $CPPFLAGS'
 ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
diff --git a/configure.ac b/configure.ac
index 4517b1c20..07ef4a40f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -249,15 +249,9 @@ AC_LANG_C
 default_cflags=no
 if test "x$CFLAGS" = "x" ; then
 ACX_CHECK_COMPILER_FLAG(g, [CFLAGS="$CFLAGS -g"])
-# ommit -O2 for clang because of optimizer failures (in expi-incep in
-# the signature serial timestamp checks.)
-if $CC --version 2>&1 | grep "clang" >/dev/null; then
-	:
-else
 ACX_CHECK_COMPILER_FLAG(O2, [CFLAGS="$CFLAGS -O2"])
 default_cflags=yes
 fi
-fi
 AC_PROG_CC
 ACX_DEPFLAG
 ACX_DETERMINE_EXT_FLAGS_UNBOUND
diff --git a/doc/Changelog b/doc/Changelog
index 9036882eb..16f3cacdb 100644
--- a/doc/Changelog
+++ b/doc/Changelog
@@ -3,6 +3,8 @@
 	  away, without a repeat query picking the DS from cache.
 	  The correct reply should have been an answer, the reply is fixed
 	  by the scrubber to have the answer in the answer section.
+	- Remove clang optimizer disable,
+	  Fix that expiration date checks don't fail with clang -O2.
 
 15 December 2017: Wouter
 	- Fix timestamp failure because of clang optimizer failure, by
diff --git a/validator/val_sigcrypt.c b/validator/val_sigcrypt.c
index 82ef6a85c..b73daf3e5 100644
--- a/validator/val_sigcrypt.c
+++ b/validator/val_sigcrypt.c
@@ -1204,14 +1204,47 @@ sigdate_error(const char* str, int32_t expi, int32_t incep, int32_t now)
 			(unsigned)incep, (unsigned)now);
 }
 
+/** RFC 1918 comparison, uses unsigned integers, and tries to avoid
+ * compiler optimization (eg. by avoiding a-b<0 comparisons),
+ * this routine matches compare_serial(), for SOA serial number checks */
+static int
+compare_1918(uint32_t a, uint32_t b)
+{
+	/* for 32 bit values */
+        const uint32_t cutoff = ((uint32_t) 1 << (32 - 1));
+
+        if (a == b) {
+                return 0;
+        } else if ((a < b && b - a < cutoff) || (a > b && a - b > cutoff)) {
+                return -1;
+        } else {
+                return 1;
+        }
+}
+
+/** if we know that b is larger than a, return the difference between them,
+ * that is the distance between them. in RFC1918 arith */
+static uint32_t
+subtract_1918(uint32_t a, uint32_t b)
+{
+	/* for 32 bit values */
+        const uint32_t cutoff = ((uint32_t) 1 << (32 - 1));
+
+	if(a == b)
+		return 0;
+	if(a < b && b - a < cutoff) {
+		return b-a;
+	}
+	return a-b;
+}
+
 /** check rrsig dates */
 static int
 check_dates(struct val_env* ve, uint32_t unow,
 	uint8_t* expi_p, uint8_t* incep_p, char** reason)
 {
 	/* read out the dates */
-	int32_t expi, incep, now;
-	int32_t incepminexpi;
+	uint32_t expi, incep, now;
 	memmove(&expi, expi_p, sizeof(expi));
 	memmove(&incep, incep_p, sizeof(incep));
 	expi = ntohl(expi);
@@ -1225,22 +1258,21 @@ check_dates(struct val_env* ve, uint32_t unow,
 		}
 		now = ve->date_override;
 		verbose(VERB_ALGO, "date override option %d", (int)now); 
-	} else	now = (int32_t)unow;
+	} else	now = unow;
 
 	/* check them */
-	incepminexpi = incep-expi;
-	if(incepminexpi > 0) {
+	if(compare_1918(incep, expi) > 0) {
 		sigdate_error("verify: inception after expiration, "
 			"signature bad", expi, incep, now);
 		*reason = "signature inception after expiration";
 		return 0;
 	}
-	if(incep - now > 0) {
+	if(compare_1918(incep, now) > 0) {
 		/* within skew ? (calc here to avoid calculation normally) */
-		int32_t skew = (expi-incep)/10;
-		if(skew < ve->skew_min) skew = ve->skew_min;
-		if(skew > ve->skew_max) skew = ve->skew_max;
-		if(incep - now > skew) {
+		uint32_t skew = subtract_1918(incep, expi)/10;
+		if(skew < (uint32_t)ve->skew_min) skew = ve->skew_min;
+		if(skew > (uint32_t)ve->skew_max) skew = ve->skew_max;
+		if(subtract_1918(now, incep) > skew) {
 			sigdate_error("verify: signature bad, current time is"
 				" before inception date", expi, incep, now);
 			*reason = "signature before inception date";
@@ -1249,11 +1281,11 @@ check_dates(struct val_env* ve, uint32_t unow,
 		sigdate_error("verify warning suspicious signature inception "
 			" or bad local clock", expi, incep, now);
 	}
-	if(now - expi > 0) {
-		int32_t skew = (expi-incep)/10;
-		if(skew < ve->skew_min) skew = ve->skew_min;
-		if(skew > ve->skew_max) skew = ve->skew_max;
-		if(now - expi > skew) {
+	if(compare_1918(now, expi) > 0) {
+		uint32_t skew = subtract_1918(incep, expi)/10;
+		if(skew < (uint32_t)ve->skew_min) skew = ve->skew_min;
+		if(skew > (uint32_t)ve->skew_max) skew = ve->skew_max;
+		if(subtract_1918(expi, now) > skew) {
 			sigdate_error("verify: signature expired", expi, 
 				incep, now);
 			*reason = "signature expired";