diff --git a/certinject/nss_internal_test.go b/certinject/nss_internal_test.go
new file mode 100644
index 0000000..2a6dc16
--- /dev/null
+++ b/certinject/nss_internal_test.go
@@ -0,0 +1,48 @@
+package certinject
+
+import (
+	"os"
+	"testing"
+	"time"
+)
+
+func TestCheckCertExpired(t *testing.T) {
+	testFilename := "test_cert_file.pem"
+
+	certExpirePeriod.SetValue(5.0)
+
+	bytesDummy := []byte(`TEST DATA`)
+
+	injectCertFile(bytesDummy, testFilename)
+	defer os.Remove(testFilename)
+
+	info1, err := os.Stat(testFilename)
+	if err != nil {
+		t.Errorf("Error getting file info 1: %s", err)
+	}
+
+	expired1, err := checkCertExpiredNss(info1)
+	if err != nil {
+		t.Errorf("Error checking if file info 1 expired: %s", err)
+	}
+
+	if expired1 {
+		t.Errorf("Cert expired instantly")
+	}
+
+	time.Sleep(10 * time.Second)
+
+	info2, err := os.Stat(testFilename)
+	if err != nil {
+		t.Errorf("Error getting file info 2: %s", err)
+	}
+
+	expired2, err := checkCertExpiredNss(info2)
+	if err != nil {
+		t.Errorf("Error checking if file info 2 expired: %s", err)
+	}
+
+	if !expired2 {
+		t.Errorf("Cert never expired")
+	}
+}