diff --git a/calendars.go b/calendars.go
index 9600be2..70890eb 100644
--- a/calendars.go
+++ b/calendars.go
@@ -4,6 +4,7 @@
 package sitemaps
 
 import (
+	"fmt"
 	"time"
 )
 
@@ -13,6 +14,10 @@ type CalendarDay struct {
 	Day   int
 }
 
+func (cd CalendarDay) ISO() string {
+	return fmt.Sprintf("%d-%02d-%02d", cd.Year, cd.Month, cd.Day)
+}
+
 func DayFrom(t time.Time) CalendarDay {
 	return CalendarDay{
 		Year:  t.Year(),
diff --git a/calendars_test.go b/calendars_test.go
index 59ff2e0..75ef772 100644
--- a/calendars_test.go
+++ b/calendars_test.go
@@ -10,7 +10,7 @@ import (
 	"github.com/shoenig/test/must"
 )
 
-func Test_DayFrom(t *testing.T) {
+func TestCalendarDay_DayFrom(t *testing.T) {
 	t.Parallel()
 
 	now := time.Date(2025, 11, 1, 6, 15, 0, 0, time.UTC)
@@ -19,3 +19,16 @@ func Test_DayFrom(t *testing.T) {
 	must.Eq(t, 11, cd.Month)
 	must.Eq(t, 1, cd.Day)
 }
+
+func TestCalendarDay_ISO(t *testing.T) {
+	t.Parallel()
+
+	cd := CalendarDay{
+		Year:  2025,
+		Month: 11,
+		Day:   1,
+	}
+
+	iso := cd.ISO()
+	must.Eq(t, "2025-11-01", iso)
+}
diff --git a/encode_test.go b/encode_test.go
new file mode 100644
index 0000000..5663cbd
--- /dev/null
+++ b/encode_test.go
@@ -0,0 +1,58 @@
+// Copyright (c) CattleCloud LLC
+// SPDX-License-Identifier: BSD-3-Clause
+
+package sitemaps
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/shoenig/test/must"
+)
+
+func TestSite_Write(t *testing.T) {
+	t.Parallel()
+
+	unix := time.Date(2025, 10, 31, 9, 15, 0, 0, time.UTC).Unix()
+	unix2 := time.Date(2025, 2, 21, 16, 30, 0, 0, time.UTC).Unix()
+
+	site := make(Site, 0, 2)
+	site = append(site, &URL{
+		Location:  "/foo/bar",
+		Modified:  EncodeUnix(unix),
+		Frequency: Daily,
+		Priority:  Bump,
+	})
+	site = append(site, &URL{
+		Location:  "/other",
+		Modified:  EncodeUnix(unix2),
+		Frequency: Weekly,
+		Priority:  None,
+	})
+
+	buf := new(bytes.Buffer)
+	err := site.Write(buf)
+	must.NoError(t, err)
+
+	exp := strings.TrimSpace(`
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+    <url>
+      <loc>/foo/bar</loc>
+      <lastmod>2025-10-31</lastmod>
+      <changefreq>daily</changefreq>
+      <priority>0.6</priority>
+    </url>
+    <url>
+      <loc>/other</loc>
+      <lastmod>2025-02-21</lastmod>
+      <changefreq>weekly</changefreq>
+      <priority>0.1</priority>
+    </url>
+</urlset>`)
+
+	result := strings.TrimSpace(buf.String())
+	must.Eq(t, exp, result)
+}
diff --git a/layout.xml b/layout.xml
index 767f27d..4830c66 100644
--- a/layout.xml
+++ b/layout.xml
@@ -3,7 +3,7 @@
   {{- range .}}
     <url>
       <loc>{{.Location}}</loc>
-      <lastmod>{{.Modified.DayISO}}</lastmod>
+      <lastmod>{{.Modified.ISO}}</lastmod>
       <changefreq>{{.Frequency}}</changefreq>
       <priority>{{.Priority}}</priority>
     </url>