(MahAppsGH-3746) Fix input for date and time text input for DateTimeP…

…icker

- if the Time is selected before a Date is selected use DateTime.Today instead of DateTime.Min
- If a new Value is selected from the Calendar we use the already Selected time.
- Implemented a new private `bool` to disable changing the time.

src/MahApps.Metro/Controls/TimePicker/DateTimePicker.cs

@@ -192,7 +192,9 @@ protected override void OnTextBoxLostFocus(object sender, RoutedEventArgs e)
             DateTime ts;
             if (DateTime.TryParse(((DatePickerTextBox)sender).Text, SpecificCultureInfo, System.Globalization.DateTimeStyles.None, out ts))
             {
+                this._deactivateAdjustTimeOnDateChange = true;
                 this.SetCurrentValue(SelectedDateTimeProperty, ts);
+                this._deactivateAdjustTimeOnDateChange = false;
             }
             else
             {

src/MahApps.Metro/Controls/TimePicker/TimePickerBase.cs

@@ -175,6 +175,7 @@ public abstract class TimePickerBase : Control
         private UIElement _secondHand;
         private Selector _secondInput;
         protected DatePickerTextBox _textBox;
+        protected bool _deactivateAdjustTimeOnDateChange;
 
         static TimePickerBase()
         {
@@ -485,7 +486,10 @@ protected virtual string GetValueForTextBox()
 
         protected virtual void OnRangeBaseValueChanged(object sender, SelectionChangedEventArgs e)
         {
-            this.SetCurrentValue(SelectedDateTimeProperty, this.SelectedDateTime.GetValueOrDefault().Date + this.GetSelectedTimeFromGUI());
+            // If we do not have a date yet we should use Today instead of DateTime.Min
+            var newDateTime = SelectedDateTime ?? DateTime.Today;
+
+            this.SetCurrentValue(SelectedDateTimeProperty, newDateTime.Date + this.GetSelectedTimeFromGUI());
         }
 
         protected virtual void OnSelectedTimeChanged(TimePickerBaseSelectionChangedEventArgs<DateTime?> e)
@@ -690,6 +694,13 @@ private static void OnSelectedDateTimeChanged(DependencyObject d, DependencyProp
                 return;
             }
 
+            // if just the DatePart changed, we should set the old TimePart for the NewValue
+            if (!timePartPickerBase._deactivateAdjustTimeOnDateChange && e.OldValue is DateTime oldVal && e.NewValue is DateTime newVal && oldVal.Date != newVal.Date && newVal.TimeOfDay.Ticks == 0 && oldVal.TimeOfDay.Ticks != 0)
+            {
+                timePartPickerBase.SetCurrentValue(SelectedDateTimeProperty, ((DateTime?)e.NewValue)?.Date + ((DateTime?)e.OldValue)?.TimeOfDay);
+                return;
+            }
+
             timePartPickerBase.SetHourPartValues((e.NewValue as DateTime?).GetValueOrDefault().TimeOfDay);
 
             timePartPickerBase.OnSelectedTimeChanged(new TimePickerBaseSelectionChangedEventArgs<DateTime?>(SelectedDateTimeChangedEvent, (DateTime?)e.OldValue, (DateTime?)e.NewValue));