[IMP] turn "Message/datetime" into field

Task-2793280

closes odoo/odoo#86448

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/components/message/message.js

@@ -8,7 +8,6 @@ import { isEventHandled, markEventHandled } from '@mail/utils/utils';
 
 import { _lt } from 'web.core';
 import Popover from "web.Popover";
-import { getLangDatetimeFormat } from 'web.time';
 
 const { Component, onWillUnmount, useRef, useState } = owl;
 
@@ -86,15 +85,6 @@ export class Message extends Component {
     // Public
     //--------------------------------------------------------------------------
 
-    /**
-     * Get the date time of the message at current user locale time.
-     *
-     * @returns {string}
-     */
-    get datetime() {
-        return this.messageView.message.date.format(getLangDatetimeFormat());
-    }
-
     /**
      * Whether the message is "active", ie: hovered or clicked, and should
      * display additional things (date in sidebar, message actions, etc.)

addons/mail/static/src/components/message/message.xml

@@ -83,7 +83,7 @@
                                     </div>
                                 </t>
                                 <t t-if="messageView.message.date">
-                                    <div class="o_Message_date o_Message_headerDate" t-att-class="{ 'o-message-selected': isSelected }" t-att-title="datetime">
+                                    <div class="o_Message_date o_Message_headerDate" t-att-class="{ 'o-message-selected': isSelected }" t-att-title="messageView.message.datetime">
                                         - <t t-esc="messageView.message.dateFromNow"/>
                                     </div>
                                 </t>

addons/mail/static/src/models/message.js

@@ -9,7 +9,7 @@ import { addLink, htmlToTextContentInline, parseAndTransform, timeFromNow } from
 import { session } from '@web/session';
 
 import { format } from 'web.field_utils';
-import { str_to_datetime } from 'web.time';
+import { getLangDatetimeFormat, str_to_datetime } from 'web.time';
 
 registerModel({
     name: 'Message',
@@ -383,6 +383,16 @@ registerModel({
             }
             return timeFromNow(this.date);
         },
+        /**
+         * @private
+         * @returns {string|FieldCommand}
+         */
+        _computeDatetime() {
+            if (!this.date) {
+                return clear();
+            }
+            return this.date.format(getLangDatetimeFormat());
+        },
         /**
          * @returns {boolean}
          */
@@ -717,6 +727,12 @@ registerModel({
         dateFromNow: attr({
             compute: '_computeDateFromNow',
         }),
+        /**
+         * The date time of the message at current user locale time.
+         */
+        datetime: attr({
+            compute: '_computeDatetime',
+        }),
         email_from: attr(),
         failureNotifications: many('Notification', {
             compute: '_computeFailureNotifications',