diff --git a/src/lib/components/schedule/BlockModal.svelte b/src/lib/components/schedule/BlockModal.svelte
index 305e61a..ea2c5e7 100644
--- a/src/lib/components/schedule/BlockModal.svelte
+++ b/src/lib/components/schedule/BlockModal.svelte
@@ -56,12 +56,15 @@
 	let step = $state<'details' | 'browse'>('details');
 	let startHour = $state('9');
 	let startMinute = $state('0');
+	let startPeriod = $state<'AM' | 'PM'>('AM');
 	let endHour = $state('10');
 	let endMinute = $state('0');
+	let endPeriod = $state<'AM' | 'PM'>('AM');
 	let selectedIds = $state<string[]>([]);
 	let error = $state('');
 
-	const hourOptions = Array.from({ length: 24 }, (_, i) => String(i));
+	const hourOptions = Array.from({ length: 12 }, (_, i) => String(i + 1));
+	const periodOptions: ('AM' | 'PM')[] = ['AM', 'PM'];
 
 	function minuteOptions(current: string): string[] {
 		const base = Array.from({ length: 60 / MINUTE_STEP }, (_, i) => String(i * MINUTE_STEP));
@@ -69,20 +72,38 @@
 		return [...base, current].sort((a, b) => Number(a) - Number(b));
 	}
 
+	function to12Hour(totalMinutes: number): { hour: number; minute: number; period: 'AM' | 'PM' } {
+		const hours24 = Math.floor(totalMinutes / 60) % 24;
+		return {
+			hour: hours24 % 12 === 0 ? 12 : hours24 % 12,
+			minute: totalMinutes % 60,
+			period: hours24 < 12 ? 'AM' : 'PM'
+		};
+	}
+
+	function to24Hour(hour12: string, minute: string, period: 'AM' | 'PM'): number {
+		const hours = (Number(hour12) % 12) + (period === 'PM' ? 12 : 0);
+		return (hours * 60 + Number(minute)) | 0;
+	}
+
 	// Re-seed the fields whenever the modal is (re)opened.
 	$effect(() => {
 		if (!open) return;
-		startHour = String(Math.floor(initialStartMinutes / 60));
-		startMinute = String(initialStartMinutes % 60);
-		endHour = String(Math.floor(initialEndMinutes / 60));
-		endMinute = String(initialEndMinutes % 60);
+		const start = to12Hour(initialStartMinutes);
+		startHour = String(start.hour);
+		startMinute = String(start.minute);
+		startPeriod = start.period;
+		const end = to12Hour(initialEndMinutes);
+		endHour = String(end.hour);
+		endMinute = String(end.minute);
+		endPeriod = end.period;
 		selectedIds = [...initialProclamationIds];
 		error = '';
 		step = 'details';
 	});
 
-	const startMinutes = $derived((Number(startHour) * 60 + Number(startMinute)) | 0);
-	const endMinutes = $derived((Number(endHour) * 60 + Number(endMinute)) | 0);
+	const startMinutes = $derived(to24Hour(startHour, startMinute, startPeriod));
+	const endMinutes = $derived(to24Hour(endHour, endMinute, endPeriod));
 	const overnight = $derived(
 		startMinutes !== endMinutes && isOvernight({ startMinutes, endMinutes })
 	);
@@ -213,9 +234,7 @@
 											class="rounded-md border border-slate-300 bg-white px-2 py-2 text-sm text-slate-900 shadow-sm focus:border-indigo-500 focus:outline-none dark:border-slate-700 dark:bg-slate-900 dark:text-slate-100"
 										>
 											{#each hourOptions as hour (hour)}
-												<option value={hour} selected={startHour === hour}>
-													{hour.padStart(2, '0')}
-												</option>
+												<option value={hour} selected={startHour === hour}>{hour}</option>
 											{/each}
 										</select>
 										<span class="text-slate-400">:</span>
@@ -230,6 +249,15 @@
 												</option>
 											{/each}
 										</select>
+										<select
+											aria-label="Start period"
+											onchange={(e) => (startPeriod = e.currentTarget.value as 'AM' | 'PM')}
+											class="rounded-md border border-slate-300 bg-white px-2 py-2 text-sm text-slate-900 shadow-sm focus:border-indigo-500 focus:outline-none dark:border-slate-700 dark:bg-slate-900 dark:text-slate-100"
+										>
+											{#each periodOptions as period (period)}
+												<option value={period} selected={startPeriod === period}>{period}</option>
+											{/each}
+										</select>
 									</div>
 								</div>
 
@@ -244,9 +272,7 @@
 											class="rounded-md border border-slate-300 bg-white px-2 py-2 text-sm text-slate-900 shadow-sm focus:border-indigo-500 focus:outline-none dark:border-slate-700 dark:bg-slate-900 dark:text-slate-100"
 										>
 											{#each hourOptions as hour (hour)}
-												<option value={hour} selected={endHour === hour}>
-													{hour.padStart(2, '0')}
-												</option>
+												<option value={hour} selected={endHour === hour}>{hour}</option>
 											{/each}
 										</select>
 										<span class="text-slate-400">:</span>
@@ -261,6 +287,15 @@
 												</option>
 											{/each}
 										</select>
+										<select
+											aria-label="End period"
+											onchange={(e) => (endPeriod = e.currentTarget.value as 'AM' | 'PM')}
+											class="rounded-md border border-slate-300 bg-white px-2 py-2 text-sm text-slate-900 shadow-sm focus:border-indigo-500 focus:outline-none dark:border-slate-700 dark:bg-slate-900 dark:text-slate-100"
+										>
+											{#each periodOptions as period (period)}
+												<option value={period} selected={endPeriod === period}>{period}</option>
+											{/each}
+										</select>
 									</div>
 								</div>
 
diff --git a/src/lib/components/schedule/BlockModal.svelte.spec.ts b/src/lib/components/schedule/BlockModal.svelte.spec.ts
index 30ef6a0..60a411b 100644
--- a/src/lib/components/schedule/BlockModal.svelte.spec.ts
+++ b/src/lib/components/schedule/BlockModal.svelte.spec.ts
@@ -67,6 +67,32 @@ describe('BlockModal', () => {
 		await expect.element(page.getByRole('combobox', { name: /^End minute$/ })).toHaveValue('15');
 	});
 
+	it('uses a 12-hour clock with an AM/PM select for each time', async () => {
+		mount({ initialStartMinutes: 21 * 60, initialEndMinutes: 0 });
+
+		await expect.element(page.getByRole('combobox', { name: /^Start hour$/ })).toHaveValue('9');
+		await expect.element(page.getByRole('combobox', { name: /^Start period$/ })).toHaveValue('PM');
+		await expect.element(page.getByRole('combobox', { name: /^End hour$/ })).toHaveValue('12');
+		await expect.element(page.getByRole('combobox', { name: /^End period$/ })).toHaveValue('AM');
+
+		// No 24-hour option exists any more.
+		await expect
+			.element(page.getByRole('combobox', { name: /^Start hour$/ }))
+			.not.toHaveValue('21');
+	});
+
+	it('recomputes the time when the AM/PM switch changes', async () => {
+		mount({ initialStartMinutes: 9 * 60, initialEndMinutes: 10 * 60 });
+
+		const period = (await page
+			.getByRole('combobox', { name: /^End period$/ })
+			.element()) as HTMLSelectElement;
+		period.value = 'PM';
+		period.dispatchEvent(new Event('change', { bubbles: true }));
+
+		await expect.element(page.getByText('9:00 AM – 10:00 PM')).toBeInTheDocument();
+	});
+
 	it('shows the review times and selected proclamation titles', async () => {
 		mount();