postit: fix rdv date picker binding

src/PostIt/PostIt.Tests/BillingCommandPageViewModelTests.cs

@@ -135,6 +135,25 @@ public class BillingCommandPageViewModelTests
         Assert.Contains("Position sélectionnée", vm.StatusMessage, StringComparison.OrdinalIgnoreCase);
     }
 
+    [Fact]
+    public void EventDateSelection_round_trips_with_EventDate_for_DatePicker_binding()
+    {
+        var api = new RecordingApi();
+        var client = new BillingApiClient(api, "https://business.example/api/v1/");
+        var vm =
+            new CommandFormSummary { Id = 12, ActionName = "Rdv", Title = "Rendez-vous" }
+            .CreateCommandPageViewModel(
+                new ActivityInfo { Code = "dev", Name = "Développement" },
+                new ActivityUserDisplayItem { PerformerId = "perf-1", UserName = "Alice" },
+                client) as RdvViewModel;
+
+        var selected = new DateTimeOffset(2026, 9, 7, 14, 30, 0, TimeSpan.FromHours(2));
+        vm!.EventDateSelection = selected;
+
+        Assert.Equal(selected.LocalDateTime, vm.EventDate);
+        Assert.Equal(vm.EventDate, vm.EventDateSelection!.Value.LocalDateTime);
+    }
+
     [Fact]
     public void ApplyResolvedAddress_populates_empty_address_directly()
     {

src/PostIt/PostIt/ViewModels/Commands/RdvViewModel.cs

@@ -33,6 +33,18 @@ public partial class RdvViewModel : BillingCommandPageViewModel
     [ObservableProperty]
     public partial DateTime EventDate { get; set; }
 
+    public DateTimeOffset? EventDateSelection
+    {
+        get => new(EventDate);
+        set
+        {
+            if (!value.HasValue)
+                return;
+
+            EventDate = value.Value.LocalDateTime;
+        }
+    }
+
     public RdvViewModel(ActivityInfo activity, ActivityUserDisplayItem performer, CommandFormSummary form, BillingApiClient billingClient)
         : base(activity, performer, form, billingClient)
     {
@@ -206,6 +218,11 @@ public partial class RdvViewModel : BillingCommandPageViewModel
         OnPropertyChanged(nameof(HasSuggestedAddressPanel));
     }
 
+    partial void OnEventDateChanged(DateTime value)
+    {
+        OnPropertyChanged(nameof(EventDateSelection));
+    }
+
 
     protected override async Task SubmitAsync()
     {

src/PostIt/PostIt/Views/Commands/RdvPage.axaml

@@ -26,7 +26,7 @@
       <TextBox Grid.Row="3" Grid.Column="1" Text="{Binding ActivityLabel}" IsReadOnly="True" Margin="0,0,0,8" />
 
       <TextBlock Grid.Row="4" Text="Date" VerticalAlignment="Center" Margin="0,0,12,8" />
-      <DatePicker Grid.Row="4" Grid.Column="1" DayFormat="ddd dd" SelectedDate="{Binding EventDate, Mode=TwoWay}" Margin="0,0,0,8" />
+      <DatePicker Grid.Row="4" Grid.Column="1" DayFormat="ddd dd" SelectedDate="{Binding EventDateSelection, Mode=TwoWay}" Margin="0,0,0,8" />
 
       <TextBlock Grid.Row="5"
                  Text="Motif"