diff --git a/webapp_calendar.py b/webapp_calendar.py
index 3f6b3d041..f68bab1df 100644
--- a/webapp_calendar.py
+++ b/webapp_calendar.py
@@ -133,9 +133,11 @@ def _html_calendar_day(person_cache: {}, css_cache: {}, translate: {},
     calendar_str += \
         '  <a href="' + cal_actor + '/calendar?year=' + str(year) + \
         '?month=' + str(month_number) + '">\n'
+    datetime_str = str(year) + '-' + str(month_number) + '-' + str(day_number)
     calendar_str += \
-        '  <h1>' + str(day_number) + ' ' + month_name + \
-        '</h1></a><br><span class="year">' + str(year) + '</span>\n'
+        '  <h1><time datetime="' + datetime_str + '">' + \
+        str(day_number) + ' ' + month_name + \
+        '</time></h1></a><br><span class="year">' + str(year) + '</span>\n'
     calendar_str += '</caption>\n'
     calendar_str += '<tbody>\n'
 
@@ -436,7 +438,10 @@ def html_calendar(person_cache: {}, css_cache: {}, translate: {},
     calendar_str += '  <a href="' + cal_actor + '/inbox" title="'
     calendar_str += translate['Switch to timeline view'] + '" ' + \
         'accesskey="' + access_keys['menuTimeline'] + '">'
-    calendar_str += '  <h1>' + month_name + '</h1></a>\n'
+    calendar_str += \
+        '  <h1><time datetime="' + \
+        str(year) + '-' + str(month_number) + '">' + month_name + \
+        '</time></h1></a>\n'
     calendar_str += \
         '  <a href="' + cal_actor + '/calendar?year=' + str(next_year) + \
         '?month=' + str(next_month_number) + '" ' + \