diff --git a/dns/dnssec.py b/dns/dnssec.py
index 7304ccc9..e989c502 100644
--- a/dns/dnssec.py
+++ b/dns/dnssec.py
@@ -23,6 +23,7 @@
 import contextlib
 import functools
 import hashlib
+import math
 import struct
 import time
 from collections.abc import Callable
@@ -37,6 +38,7 @@
 import dns.rdataset
 import dns.rdatatype
 import dns.rrset
+import dns.serial
 import dns.transaction
 import dns.zone
 from dns.dnssectypes import Algorithm, DSDigest, NSEC3Hash
@@ -369,9 +371,18 @@ def _validate_rrsig(
 
     if now is None:
         now = time.time()
-    if rrsig.expiration < now:
+    try:
+        now_seconds = math.floor(now)
+    except (OverflowError, ValueError) as exc:
+        raise ValidationFailure("invalid validation time") from exc
+
+    # RFC 4034 section 3.1.5: compare RRSIG times as 32-bit serial numbers.
+    current = dns.serial.Serial(now_seconds)
+    expiration = dns.serial.Serial(rrsig.expiration)
+    # The exact expiration instant is inclusive, even when now is fractional.
+    if not (current <= expiration) or (current == expiration and now > now_seconds):
         raise ValidationFailure("expired")
-    if rrsig.inception > now:
+    if not (dns.serial.Serial(rrsig.inception) <= current):
         raise ValidationFailure("not yet valid")
 
     data = _make_rrsig_signature_data(rrset, rrsig, origin)
diff --git a/tests/test_dnssec.py b/tests/test_dnssec.py
index e69bebcd..e744d093 100644
--- a/tests/test_dnssec.py
+++ b/tests/test_dnssec.py
@@ -1089,6 +1089,62 @@ def testTimeBounds(self):
                 abs_ed448_mx, abs_ed448_mx_rrsig_1, abs_ed448_keys_1, None
             )
 
+    def testTimeBoundsAcrossRollover(self) -> None:
+        key = ed25519.Ed25519PrivateKey.generate()
+        signer = abs_soa.name
+        dnskey = dns.dnssec.make_dnskey(key.public_key(), dns.dnssec.Algorithm.ED25519)
+        keys = {signer: dns.rrset.from_rdata(signer, 300, dnskey)}
+        rollover = 2**32
+        rrsig = dns.dnssec.sign(
+            abs_soa, key, signer, dnskey, inception=rollover - 16, expiration=32
+        )
+
+        for now in (rollover - 16, rollover - 1, rollover + 1, rollover + 32):
+            with self.subTest(now=now):
+                dns.dnssec.validate_rrsig(abs_soa, rrsig, keys, now=now)
+
+        for now, message in (
+            (rollover - 16.5, "not yet valid"),
+            (rollover + 32.5, "expired"),
+            (float("nan"), "invalid validation time"),
+            (float("inf"), "invalid validation time"),
+        ):
+            with self.subTest(now=now):
+                with self.assertRaisesRegex(dns.dnssec.ValidationFailure, message):
+                    dns.dnssec.validate_rrsig(abs_soa, rrsig, keys, now=now)
+
+    def testTimeBoundsAtSerialHalfRange(self) -> None:
+        key = ed25519.Ed25519PrivateKey.generate()
+        signer = abs_soa.name
+        dnskey = dns.dnssec.make_dnskey(key.public_key(), dns.dnssec.Algorithm.ED25519)
+        keys = {signer: dns.rrset.from_rdata(signer, 300, dnskey)}
+        half_range = 2**31
+        serial_space = 2**32
+        now = half_range + 100
+
+        # RFC 1982 leaves ordering undefined at exactly half the serial space.
+        # Keep one bound equal to now while testing the other bound.
+        for inception, expiration, message in (
+            (now - half_range + 1, now, None),
+            (now - half_range, now, "not yet valid"),
+            (now, now + half_range - 1, None),
+            (now, now + half_range, "expired"),
+        ):
+            with self.subTest(inception=inception, expiration=expiration):
+                rrsig = dns.dnssec.sign(
+                    abs_soa,
+                    key,
+                    signer,
+                    dnskey,
+                    inception=inception % serial_space,
+                    expiration=expiration % serial_space,
+                )
+                if message is None:
+                    dns.dnssec.validate_rrsig(abs_soa, rrsig, keys, now=now)
+                else:
+                    with self.assertRaisesRegex(dns.dnssec.ValidationFailure, message):
+                        dns.dnssec.validate_rrsig(abs_soa, rrsig, keys, now=now)
+
     def testOwnerNameMismatch(self):
         bogus = dns.name.from_text("example.bogus")
         with self.assertRaises(dns.dnssec.ValidationFailure):