diff --git a/src/app-state.ts b/src/app-state.ts
index fe09c36..8b0beb3 100644
--- a/src/app-state.ts
+++ b/src/app-state.ts
@@ -16,9 +16,22 @@ export function todayIso(): string {
   return `${yyyy}-${mm}-${dd}`;
 }
 
+/** Which hour to default to when the URL doesn't specify one, based on local wall-clock time. */
+export function defaultHourForTime(date: Date): HourId {
+  const h = date.getHours();
+  if (h < 6) return 'matins';
+  if (h < 8) return 'lauds';
+  if (h < 9) return 'prime';
+  if (h < 12) return 'terce';
+  if (h < 15) return 'sext';
+  if (h < 17) return 'none';
+  if (h < 21) return 'vespers';
+  return 'compline';
+}
+
 const state: AppState = {
   date: todayIso(),
-  selectedHour: 'prime',
+  selectedHour: defaultHourForTime(new Date()),
   languages: ['en'],
 };
 
diff --git a/tests/app-state.test.ts b/tests/app-state.test.ts
new file mode 100644
index 0000000..ab39cd7
--- /dev/null
+++ b/tests/app-state.test.ts
@@ -0,0 +1,39 @@
+import { describe, it, expect } from 'vitest';
+import { defaultHourForTime } from '../src/app-state';
+
+function at(hour: number, minute = 0): Date {
+  const d = new Date();
+  d.setHours(hour, minute, 0, 0);
+  return d;
+}
+
+describe('defaultHourForTime', () => {
+  it.each([
+    [0, 30, 'matins'],
+    [3, 0, 'matins'],
+    [5, 59, 'matins'],
+    [6, 0, 'lauds'],
+    [6, 30, 'lauds'],
+    [7, 59, 'lauds'],
+    [8, 0, 'prime'],
+    [8, 30, 'prime'],
+    [8, 59, 'prime'],
+    [9, 0, 'terce'],
+    [10, 0, 'terce'],
+    [11, 59, 'terce'],
+    [12, 0, 'sext'],
+    [13, 0, 'sext'],
+    [14, 59, 'sext'],
+    [15, 0, 'none'],
+    [16, 0, 'none'],
+    [16, 59, 'none'],
+    [17, 0, 'vespers'],
+    [18, 0, 'vespers'],
+    [20, 59, 'vespers'],
+    [21, 0, 'compline'],
+    [22, 0, 'compline'],
+    [23, 59, 'compline'],
+  ] as const)('%i:%i -> %s', (hour, minute, expected) => {
+    expect(defaultHourForTime(at(hour, minute))).toBe(expected);
+  });
+});