diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0c940e6..0683f1c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 All notable changes to this project will be documented in this file.
 
+## 2.6.6 - 2026-08-07
+
+### Fixed
+
+- Timetable entry editing now validates end times locally, requires the end to be at least one minute after the start, and highlights the end field when the value is invalid.
+
 ## 2.6.5 - 2026-08-07
 
 ### Added
diff --git a/package.json b/package.json
index cb2960b..4556c1a 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "pulse-signage",
-  "version": "2.6.5",
+  "version": "2.6.6",
   "private": false,
   "description": "Pulse Signage application with MySQL and media storage",
   "repository": {
diff --git a/src/web/public/js/data-sources/timetable-group-form.js b/src/web/public/js/data-sources/timetable-group-form.js
index 0a0322d..69b9778 100644
--- a/src/web/public/js/data-sources/timetable-group-form.js
+++ b/src/web/public/js/data-sources/timetable-group-form.js
@@ -47,6 +47,101 @@
     return Number.isNaN(date.getTime()) ? null : date;
   }
 
+  function parseDateTimeLocalValue(value) {
+    var raw = String(value || '').trim();
+    if (!raw) {
+      return null;
+    }
+
+    var date = new Date(raw);
+    return Number.isNaN(date.getTime()) ? null : date;
+  }
+
+  function clearFieldValidity(input) {
+    if (!input) {
+      return;
+    }
+
+    input.setCustomValidity('');
+    input.classList.remove('is-invalid');
+    if (input.removeAttribute) {
+      input.removeAttribute('aria-invalid');
+    }
+  }
+
+  function setFieldValidity(input, message) {
+    if (!input) {
+      return;
+    }
+
+    input.setCustomValidity(message);
+    input.classList.add('is-invalid');
+    if (input.setAttribute) {
+      input.setAttribute('aria-invalid', 'true');
+    }
+  }
+
+  function validateEntryRow(row) {
+    if (!row) {
+      return;
+    }
+
+    var startInput = row.querySelector('[name="entry_start_datetime[]"]');
+    var endInput = row.querySelector('[name="entry_end_datetime[]"]');
+    var startValue;
+    var endValue;
+    var startDate;
+    var endDate;
+
+    clearFieldValidity(endInput);
+
+    if (!startInput || !endInput) {
+      return;
+    }
+
+    startValue = String(startInput.value || '').trim();
+    endValue = String(endInput.value || '').trim();
+
+    if (!startValue || !endValue) {
+      return;
+    }
+
+    startDate = parseDateTimeLocalValue(startValue);
+    endDate = parseDateTimeLocalValue(endValue);
+
+    if (!startDate || !endDate) {
+      return;
+    }
+
+    if (endDate.getTime() < startDate.getTime() + 60000) {
+      setFieldValidity(endInput, 'End time must be at least 1 minute after the start time.');
+    }
+  }
+
+  function bindRowValidation(row) {
+    if (!row) {
+      return;
+    }
+
+    var startInput = row.querySelector('[name="entry_start_datetime[]"]');
+    var endInput = row.querySelector('[name="entry_end_datetime[]"]');
+
+    if (!startInput || !endInput) {
+      return;
+    }
+
+    function handleValidation() {
+      validateEntryRow(row);
+    }
+
+    startInput.addEventListener('input', handleValidation);
+    startInput.addEventListener('change', handleValidation);
+    endInput.addEventListener('input', handleValidation);
+    endInput.addEventListener('change', handleValidation);
+
+    validateEntryRow(row);
+  }
+
   function toUtcDateTimeLocalValue(value) {
     var localDate = new Date(String(value || '').trim());
     return Number.isNaN(localDate.getTime()) ? '' : localDate.toISOString();
@@ -110,6 +205,7 @@
       return;
     }
     bindRemove(row);
+    bindRowValidation(row);
     body.appendChild(fragment);
     markDirty();
   }
@@ -117,6 +213,7 @@
   body.querySelectorAll('[data-timetable-entry-row]').forEach(function (row) {
     bindRemove(row);
     syncRowValuesToLocal(row);
+    bindRowValidation(row);
   });
 
   form.addEventListener('formdata', function (event) {
diff --git a/src/web/views/data-sources/timetables/form.hbs b/src/web/views/data-sources/timetables/form.hbs
index 6bfdb0c..63081e9 100644
--- a/src/web/views/data-sources/timetables/form.hbs
+++ b/src/web/views/data-sources/timetables/form.hbs
@@ -59,6 +59,7 @@
                 </td>
                 <td>
                   <input type="datetime-local" name="entry_end_datetime[]" class="form-control" value="{{endValue}}" />
+                  <div class="invalid-feedback">End time must be at least 1 minute after the start time.</div>
                 </td>
                 <td>
                   <button type="button" class="btn btn-outline-danger btn-sm" data-remove-timetable-entry>Remove</button>
@@ -86,6 +87,7 @@
     </td>
     <td>
       <input type="datetime-local" name="entry_end_datetime[]" class="form-control" value="" />
+      <div class="invalid-feedback">End time must be at least 1 minute after the start time.</div>
     </td>
     <td>
       <button type="button" class="btn btn-outline-danger btn-sm" data-remove-timetable-entry>Remove</button>
diff --git a/test/timetable-group-form.test.js b/test/timetable-group-form.test.js
new file mode 100644
index 0000000..ec59947
--- /dev/null
+++ b/test/timetable-group-form.test.js
@@ -0,0 +1,207 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const vm = require('node:vm');
+
+function createClassList() {
+  const classes = new Set();
+
+  return {
+    add(name) {
+      classes.add(name);
+    },
+    remove(name) {
+      classes.delete(name);
+    },
+    contains(name) {
+      return classes.has(name);
+    }
+  };
+}
+
+function createInput(name, value) {
+  const listeners = Object.create(null);
+  const attributes = Object.create(null);
+
+  return {
+    name: name,
+    value: value || '',
+    dataset: {},
+    classList: createClassList(),
+    attributes: attributes,
+    validityMessage: '',
+    addEventListener(type, handler) {
+      if (!listeners[type]) {
+        listeners[type] = [];
+      }
+      listeners[type].push(handler);
+    },
+    dispatchEvent(event) {
+      const handlers = listeners[event.type] || [];
+      handlers.forEach(function (handler) {
+        handler.call(this, event);
+      }, this);
+    },
+    setCustomValidity(message) {
+      this.validityMessage = String(message || '');
+    },
+    setAttribute(name, value) {
+      attributes[name] = String(value);
+    },
+    removeAttribute(name) {
+      delete attributes[name];
+    }
+  };
+}
+
+function createRow(startValue, endValue) {
+  const startInput = createInput('entry_start_datetime[]', startValue);
+  const endInput = createInput('entry_end_datetime[]', endValue);
+  const titleInput = createInput('entry_title[]', 'Opening');
+  const descriptionInput = createInput('entry_short_description[]', '');
+  const idInput = createInput('entry_id[]', '1');
+  const removeButton = {
+    addEventListener() {}
+  };
+
+  const row = {
+    startInput: startInput,
+    endInput: endInput,
+    querySelector(selector) {
+      if (selector === '[name="entry_start_datetime[]"]') {
+        return startInput;
+      }
+      if (selector === '[name="entry_end_datetime[]"]') {
+        return endInput;
+      }
+      if (selector === '[name="entry_title[]"]') {
+        return titleInput;
+      }
+      if (selector === '[name="entry_short_description[]"]') {
+        return descriptionInput;
+      }
+      if (selector === '[name="entry_id[]"]') {
+        return idInput;
+      }
+      if (selector === '[data-remove-timetable-entry]') {
+        return removeButton;
+      }
+      return null;
+    },
+    remove() {
+      row.removed = true;
+    }
+  };
+
+  return row;
+}
+
+function formatDateTimeLocalValue(date) {
+  function pad(value) {
+    return String(value).padStart(2, '0');
+  }
+
+  return [
+    String(date.getFullYear()).padStart(4, '0'),
+    '-',
+    pad(date.getMonth() + 1),
+    '-',
+    pad(date.getDate()),
+    'T',
+    pad(date.getHours()),
+    ':',
+    pad(date.getMinutes())
+  ].join('');
+}
+
+test('timetable group form highlights an end time that is too early', () => {
+  const row = createRow('2026-08-07T10:00', '2026-08-07T10:00');
+  const body = {
+    rows: [row],
+    querySelectorAll(selector) {
+      return selector === '[data-timetable-entry-row]' ? this.rows.slice() : [];
+    },
+    appendChild(node) {
+      this.rows.push(node);
+      return node;
+    }
+  };
+  const addButton = {
+    listeners: Object.create(null),
+    addEventListener(type, handler) {
+      this.listeners[type] = handler;
+    },
+    click() {
+      if (this.listeners.click) {
+        this.listeners.click();
+      }
+    }
+  };
+  const form = {
+    dataset: {},
+    addEventListener() {}
+  };
+  const templateRow = createRow('', '');
+  const template = {
+    content: {
+      cloneNode() {
+        return {
+          querySelector(selector) {
+            return selector === '[data-timetable-entry-row]' ? templateRow : null;
+          }
+        };
+      }
+    }
+  };
+
+  const sandbox = {
+    document: {
+      getElementById(id) {
+        if (id === 'timetable-group-form') {
+          return form;
+        }
+        if (id === 'timetable-entry-row-template') {
+          return template;
+        }
+        return null;
+      },
+      querySelector(selector) {
+        if (selector === '[data-timetable-entries-body]') {
+          return body;
+        }
+        if (selector === '[data-add-timetable-entry]') {
+          return addButton;
+        }
+        return null;
+      }
+    },
+    window: {},
+    Date: Date,
+    Number: Number,
+    String: String,
+    Array: Array,
+    Object: Object,
+    Math: Math,
+    JSON: JSON,
+    console: console,
+    module: { exports: {} },
+    exports: {}
+  };
+  sandbox.window = sandbox;
+
+  const scriptPath = path.join(__dirname, '..', 'src', 'web', 'public', 'js', 'data-sources', 'timetable-group-form.js');
+  const script = fs.readFileSync(scriptPath, 'utf8');
+  vm.runInNewContext(script, sandbox, { filename: scriptPath });
+
+  assert.equal(row.endInput.validityMessage, 'End time must be at least 1 minute after the start time.');
+  assert.equal(row.endInput.classList.contains('is-invalid'), true);
+  assert.equal(row.endInput.attributes['aria-invalid'], 'true');
+
+  row.endInput.value = formatDateTimeLocalValue(new Date(new Date(row.startInput.value).getTime() + 60000));
+  row.endInput.dispatchEvent({ type: 'input' });
+
+  assert.equal(row.endInput.validityMessage, '');
+  assert.equal(row.endInput.classList.contains('is-invalid'), false);
+  assert.equal(Object.prototype.hasOwnProperty.call(row.endInput.attributes, 'aria-invalid'), false);
+});
\ No newline at end of file