diff --git a/lib/screens/professional/professional_calendar_screen.dart b/lib/screens/professional/professional_calendar_screen.dart
index bae7a34..503d083 100644
--- a/lib/screens/professional/professional_calendar_screen.dart
+++ b/lib/screens/professional/professional_calendar_screen.dart
@@ -6,6 +6,7 @@ import 'package:intl/intl.dart';
 import 'package:professional_repository/professional_repository.dart';
 import 'package:prosappco/blocs/service_bloc/service_bloc.dart';
 import 'package:prosappco/screens/service/professional_service_screen.dart';
+import 'package:prosappco/utils/service_day.dart';
 import 'package:prosappco/utils/time_of_day_extension.dart';
 import 'package:prosappco/utils/time_of_day_utils.dart';
 import 'package:service_repository/service_repository.dart';
@@ -596,7 +597,7 @@ class _ProfessionalCalendarScreenState
       TimeOfDay time, List<ServiceEntity>? services, DateTime day) {
     if (services == null) return null;
     for (final s in services) {
-      if (day.toString() != s.day || s.range1Hour1 != time) continue;
+      if (!isSameServiceDay(day, s.day) || s.range1Hour1 != time) continue;
       if (s.status == ServiceStatus.cancelled ||
           s.status == ServiceStatus.denied) {
         continue;
diff --git a/lib/screens/user/user_calendar_screen.dart b/lib/screens/user/user_calendar_screen.dart
index b63fba4..97a1205 100644
--- a/lib/screens/user/user_calendar_screen.dart
+++ b/lib/screens/user/user_calendar_screen.dart
@@ -5,6 +5,7 @@ import 'package:intl/intl.dart';
 import 'package:professional_repository/professional_repository.dart';
 import 'package:prosappco/blocs/professional_list_bloc/professional_list_bloc.dart';
 import 'package:prosappco/utils/time_of_day_extension.dart';
+import 'package:prosappco/utils/service_day.dart';
 import 'package:prosappco/utils/time_of_day_utils.dart';
 import 'package:service_repository/service_repository.dart';
 import 'package:setting_repository/setting_repository.dart';
@@ -233,14 +234,16 @@ class UserCalendarScreenState extends State<UserCalendarScreen> {
 
   bool _isHora1Ocupada(
       TimeOfDay hora1, List<ServiceEntity>? events, DateTime selectedDay) {
-    if (events != null) {
-      for (ServiceEntity event in events) {
-        if (selectedDay.toString() == event.day) {
-          if (hora1 == event.range1Hour1) {
-            return true;
-          }
-        }
+    // Treat "could not load the agenda" as busy rather than free: showing a
+    // taken slot as available leads straight to a double booking.
+    if (events == null) return true;
+    for (final event in events) {
+      if (!isSameServiceDay(selectedDay, event.day)) continue;
+      if (event.status == ServiceStatus.cancelled ||
+          event.status == ServiceStatus.denied) {
+        continue;
       }
+      if (hora1 == event.range1Hour1) return true;
     }
     return false;
   }
diff --git a/lib/utils/service_day.dart b/lib/utils/service_day.dart
new file mode 100644
index 0000000..2e9fcfa
--- /dev/null
+++ b/lib/utils/service_day.dart
@@ -0,0 +1,14 @@
+/// Whether a service's `day` field refers to the same calendar day as [day].
+///
+/// The backend returns ISO 8601 ("2026-07-13T00:00:00.000Z") while
+/// `DateTime.toString()` produces a space separator ("2026-07-13 00:00:00.000Z"),
+/// so comparing the two as raw strings never matched and every booked slot
+/// rendered as free. Compare parsed calendar dates instead.
+bool isSameServiceDay(DateTime day, String? serviceDay) {
+  if (serviceDay == null || serviceDay.isEmpty) return false;
+  final parsed = DateTime.tryParse(serviceDay);
+  if (parsed == null) return false;
+  return parsed.year == day.year &&
+      parsed.month == day.month &&
+      parsed.day == day.day;
+}