forbid changing year when editing existing logbook entry

templates/logbookform.html

@@ -32,7 +32,7 @@
            <input name="prev_slug" id="prev_slug" value="{{slug}}" type=hidden>
 
            <span {% if dateflag %}style="color:red"{% endif %}>
-           <label for="date">Date of the activity</label>
+           <label for="date">Date of the activity{% if dateflag %} (cannot change year on an existing entry){% endif %}</label>
            <input {% if not user.username %} disabled{% endif %} 
            label = "Date" name = "date" size="12"
            title="Date of the activity, a single day,  in ISO format: 2020-08-17"