diff --git a/application/Espo/Classes/FieldProcessing/WorkingTimeCalendar/IsDefaultLoader.php b/application/Espo/Classes/FieldProcessing/WorkingTimeCalendar/IsDefaultLoader.php
new file mode 100644
index 0000000000..0d1b11e8a9
--- /dev/null
+++ b/application/Espo/Classes/FieldProcessing/WorkingTimeCalendar/IsDefaultLoader.php
@@ -0,0 +1,53 @@
+<?php
+/************************************************************************
+ * This file is part of EspoCRM.
+ *
+ * EspoCRM – Open Source CRM application.
+ * Copyright (C) 2014-2026 EspoCRM, Inc.
+ * Website: https://www.espocrm.com
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+ *
+ * The interactive user interfaces in modified source and object code versions
+ * of this program must display Appropriate Legal Notices, as required under
+ * Section 5 of the GNU Affero General Public License version 3.
+ *
+ * In accordance with Section 7(b) of the GNU Affero General Public License version 3,
+ * these Appropriate Legal Notices must retain the display of the "EspoCRM" word.
+ ************************************************************************/
+
+namespace Espo\Classes\FieldProcessing\WorkingTimeCalendar;
+
+use Espo\Core\FieldProcessing\Loader;
+use Espo\Core\FieldProcessing\Loader\Params;
+use Espo\Core\Utils\Config;
+use Espo\Entities\WorkingTimeCalendar;
+use Espo\ORM\Entity;
+
+/**
+ * @implements Loader<WorkingTimeCalendar>
+ */
+class IsDefaultLoader implements Loader
+{
+    public function __construct(
+        private Config $config,
+    ) {}
+
+    public function process(Entity $entity, Params $params): void
+    {
+        $isDefault = $entity->getId() === $this->config->get('workingTimeCalendarId');
+
+        $entity->set('isDefault', $isDefault);
+    }
+}
diff --git a/application/Espo/Resources/i18n/en_US/WorkingTimeCalendar.json b/application/Espo/Resources/i18n/en_US/WorkingTimeCalendar.json
index 960e132ecf..3df8bbc930 100644
--- a/application/Espo/Resources/i18n/en_US/WorkingTimeCalendar.json
+++ b/application/Espo/Resources/i18n/en_US/WorkingTimeCalendar.json
@@ -18,9 +18,14 @@
         "weekday3TimeRanges": "Wed Schedule",
         "weekday4TimeRanges": "Thu Schedule",
         "weekday5TimeRanges": "Fri Schedule",
-        "weekday6TimeRanges": "Sat Schedule"
+        "weekday6TimeRanges": "Sat Schedule",
+        "isDefault": "Is Default"
     },
     "links": {
+        "users": "Users",
         "ranges": "Exceptions"
+    },
+    "tooltips": {
+        "isDefault": "Indicates whether this calendar is the system default. The default working time calendar can be configured in: Administration > Settings > Working Time Calendar."
     }
 }
diff --git a/application/Espo/Resources/layouts/WorkingTimeCalendar/defaultSidePanel.json b/application/Espo/Resources/layouts/WorkingTimeCalendar/defaultSidePanel.json
index fe51488c70..e641ce2505 100644
--- a/application/Espo/Resources/layouts/WorkingTimeCalendar/defaultSidePanel.json
+++ b/application/Espo/Resources/layouts/WorkingTimeCalendar/defaultSidePanel.json
@@ -1 +1,3 @@
-[]
+[
+    "isDefault"
+]
diff --git a/application/Espo/Resources/layouts/WorkingTimeCalendar/list.json b/application/Espo/Resources/layouts/WorkingTimeCalendar/list.json
index ac1822864f..602f42bfbf 100644
--- a/application/Espo/Resources/layouts/WorkingTimeCalendar/list.json
+++ b/application/Espo/Resources/layouts/WorkingTimeCalendar/list.json
@@ -2,5 +2,9 @@
     {
         "name": "name",
         "link": true
+    },
+    {
+        "name": "isDefault",
+        "width": 14
     }
 ]
diff --git a/application/Espo/Resources/layouts/WorkingTimeCalendar/relationships.json b/application/Espo/Resources/layouts/WorkingTimeCalendar/relationships.json
index 4cbbe81f7c..cdca22c0cd 100644
--- a/application/Espo/Resources/layouts/WorkingTimeCalendar/relationships.json
+++ b/application/Espo/Resources/layouts/WorkingTimeCalendar/relationships.json
@@ -1,3 +1,5 @@
 [
-    "ranges"
+    "ranges",
+    "users",
+    "teams"
 ]
diff --git a/application/Espo/Resources/metadata/clientDefs/WorkingTimeCalendar.json b/application/Espo/Resources/metadata/clientDefs/WorkingTimeCalendar.json
index d9eb0151e8..b03566d01c 100644
--- a/application/Espo/Resources/metadata/clientDefs/WorkingTimeCalendar.json
+++ b/application/Espo/Resources/metadata/clientDefs/WorkingTimeCalendar.json
@@ -15,5 +15,17 @@
                 }
             ]
         }
+    },
+    "relationshipPanels": {
+        "teams": {
+            "createDisabled": true,
+            "editDisabled": true,
+            "removeDisabled": true
+        },
+        "users": {
+            "createDisabled": true,
+            "editDisabled": true,
+            "removeDisabled": true
+        }
     }
 }
diff --git a/application/Espo/Resources/metadata/entityDefs/WorkingTimeCalendar.json b/application/Espo/Resources/metadata/entityDefs/WorkingTimeCalendar.json
index ab4a0f2562..7bcaf4e570 100644
--- a/application/Espo/Resources/metadata/entityDefs/WorkingTimeCalendar.json
+++ b/application/Espo/Resources/metadata/entityDefs/WorkingTimeCalendar.json
@@ -83,6 +83,14 @@
             "default": null,
             "view": "views/working-time-calendar/fields/time-ranges"
         },
+        "isDefault": {
+            "type": "bool",
+            "readOnly": true,
+            "notStorable": true,
+            "orderDisabled": true,
+            "loaderClassName": "Espo\\Classes\\FieldProcessing\\WorkingTimeCalendar\\IsDefaultLoader",
+            "tooltip": true
+        },
         "teams": {
             "type": "linkMultiple",
             "readOnly": true
@@ -110,6 +118,12 @@
             "foreign": "calendars",
             "entity": "WorkingTimeRange"
         },
+        "users": {
+            "type": "hasMany",
+            "foreign": "workingTimeCalendar",
+            "entity": "User",
+            "readOnly": true
+        },
         "teams": {
             "type": "hasMany",
             "foreign": "workingTimeCalendar",
diff --git a/application/Espo/Resources/metadata/logicDefs/WorkingTimeCalendar.json b/application/Espo/Resources/metadata/logicDefs/WorkingTimeCalendar.json
index fdccd8ba44..fa5894b0a8 100644
--- a/application/Espo/Resources/metadata/logicDefs/WorkingTimeCalendar.json
+++ b/application/Espo/Resources/metadata/logicDefs/WorkingTimeCalendar.json
@@ -79,6 +79,16 @@
                     }
                 ]
             }
+        },
+        "isDefault": {
+            "visible": {
+                "conditionGroup": [
+                    {
+                        "type": "isNotEmpty",
+                        "attribute": "id"
+                    }
+                ]
+            }
         }
     }
 }