diff --git a/pkg/verify/certificate.go b/pkg/verify/certificate.go
index 843d6ccc..5ee4561e 100644
--- a/pkg/verify/certificate.go
+++ b/pkg/verify/certificate.go
@@ -24,10 +24,10 @@ import (
 
 func VerifyLeafCertificate(observerTimestamp time.Time, leafCert x509.Certificate, trustedMaterial root.TrustedMaterial) error { // nolint: revive
 	for _, ca := range trustedMaterial.FulcioCertificateAuthorities() {
-		if !ca.ValidityPeriodStart.IsZero() && leafCert.NotBefore.Before(ca.ValidityPeriodStart) {
+		if !ca.ValidityPeriodStart.IsZero() && observerTimestamp.Before(ca.ValidityPeriodStart) {
 			continue
 		}
-		if !ca.ValidityPeriodEnd.IsZero() && leafCert.NotAfter.After(ca.ValidityPeriodEnd) {
+		if !ca.ValidityPeriodEnd.IsZero() && observerTimestamp.After(ca.ValidityPeriodEnd) {
 			continue
 		}
 
diff --git a/pkg/verify/certificate_test.go b/pkg/verify/certificate_test.go
new file mode 100644
index 00000000..6603f06f
--- /dev/null
+++ b/pkg/verify/certificate_test.go
@@ -0,0 +1,61 @@
+// Copyright 2023 The Sigstore Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package verify_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/sigstore/sigstore-go/pkg/testing/ca"
+	"github.com/sigstore/sigstore-go/pkg/verify"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestVerifyValidityPeriod(t *testing.T) {
+	virtualSigstore, err := ca.NewVirtualSigstore()
+	assert.NoError(t, err)
+
+	leaf, _, err := virtualSigstore.GenerateLeafCert("example@example.com", "issuer")
+	assert.NoError(t, err)
+
+	tests := []struct {
+		name              string
+		observerTimestamp time.Time
+		wantErr           bool
+	}{
+		{
+			name:              "before validity period",
+			observerTimestamp: time.Now().Add(time.Hour * -24),
+			wantErr:           true,
+		},
+		{
+			name:              "inside validity period",
+			observerTimestamp: time.Now(),
+			wantErr:           false,
+		},
+		{
+			name:              "after validity period",
+			observerTimestamp: time.Now().Add(time.Hour * 24),
+			wantErr:           true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := verify.VerifyLeafCertificate(tt.observerTimestamp, *leaf, virtualSigstore); (err != nil) != tt.wantErr {
+				t.Errorf("VerifyLeafCertificate() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}