allow to select a date & fix issue

Billing.Shared/Helper.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Globalization;
 using Xamarin.Essentials;
+using Xamarin.Forms;
 
 namespace Billing
 {
@@ -73,5 +74,45 @@ namespace Billing
             }
             return color;
         }
+
+        public static bool IsSameDay(DateTime day1, DateTime day2)
+        {
+            return day1.Year == day2.Year && day1.DayOfYear == day2.DayOfYear;
+        }
+
+        public static DateTime FirstDay(DateTime date)
+        {
+            var week = date.DayOfWeek;
+            if (week == DayOfWeek.Sunday)
+            {
+                return date;
+            }
+            return date.AddDays(-(int)week);
+        }
+
+        public static BindableProperty Create<TResult, TOwner>(string name, TResult defaultValue = default, PropertyValueChanged<TResult, TOwner> propertyChanged = null)
+            where TOwner : BindableObject
+        {
+            if (propertyChanged == null)
+            {
+                return BindableProperty.Create(name, typeof(TResult), typeof(TOwner), defaultValue: defaultValue);
+            }
+            return BindableProperty.Create(name, typeof(TResult), typeof(TOwner),
+                defaultValue: defaultValue,
+                propertyChanged: (obj, old, @new) =>
+                {
+                    if (old != null && !(old is TResult))
+                    {
+                        return;
+                    }
+                    if (@new != null && !(@new is TResult))
+                    {
+                        return;
+                    }
+                    propertyChanged((TOwner)obj, (TResult)old, (TResult)@new);
+                });
+        }
+
+        public delegate void PropertyValueChanged<TResult, TOwner>(TOwner obj, TResult old, TResult @new);
     }
 }