Cleanup some DateTime related constants (#105071)

src/libraries/System.Private.CoreLib/src/System/DateOnly.cs

@@ -26,8 +26,8 @@ public readonly struct DateOnly
         // Maps to Jan 1st year 1
         private const int MinDayNumber = 0;
 
-        // Maps to December 31 year 9999. The value calculated from "new DateTime(9999, 12, 31).Ticks / TimeSpan.TicksPerDay"
-        private const int MaxDayNumber = 3_652_058;
+        // Maps to December 31 year 9999.
+        private const int MaxDayNumber = DateTime.DaysTo10000 - 1;
 
         private static int DayNumberFromDateTime(DateTime dt) => (int)((ulong)dt.Ticks / TimeSpan.TicksPerDay);
 

src/libraries/System.Private.CoreLib/src/System/DateTime.cs

@@ -72,7 +72,7 @@ public readonly partial struct DateTime
         // Number of days from 1/1/0001 to 12/31/1969
         internal const int DaysTo1970 = DaysPer400Years * 4 + DaysPer100Years * 3 + DaysPer4Years * 17 + DaysPerYear; // 719,162
         // Number of days from 1/1/0001 to 12/31/9999
-        private const int DaysTo10000 = DaysPer400Years * 25 - 366;  // 3652059
+        internal const int DaysTo10000 = DaysPer400Years * 25 - 366;  // 3652059
 
         internal const long MinTicks = 0;
         internal const long MaxTicks = DaysTo10000 * TimeSpan.TicksPerDay - 1;

src/libraries/System.Private.CoreLib/src/System/Globalization/Calendar.cs

@@ -28,19 +28,7 @@ namespace System.Globalization
 
     public abstract class Calendar : ICloneable
     {
-        // Number of days in a non-leap year
-        internal const int DaysPerYear = 365;
-        // Number of days in 4 years
-        internal const int DaysPer4Years = DaysPerYear * 4 + 1;
-        // Number of days in 100 years
-        internal const int DaysPer100Years = DaysPer4Years * 25 - 1;
-        // Number of days in 400 years
-        internal const int DaysPer400Years = DaysPer100Years * 4 + 1;
-
-        // Number of days from 1/1/0001 to 1/1/10000
-        internal const int DaysTo10000 = DaysPer400Years * 25 - 366;
-
-        internal const long MaxMillis = (long)DaysTo10000 * TimeSpan.MillisecondsPerDay;
+        internal const long MaxMillis = (long)DateTime.DaysTo10000 * TimeSpan.MillisecondsPerDay;
 
         private int _currentEraValue = -1;
 

src/libraries/System.Private.CoreLib/src/System/TimeOnly.cs

@@ -26,8 +26,8 @@ public readonly struct TimeOnly
         // MinTimeTicks is the ticks for the midnight time 00:00:00.000 AM
         private const long MinTimeTicks = 0;
 
-        // MaxTimeTicks is the max tick value for the time in the day. It is calculated using DateTime.Today.AddTicks(-1).TimeOfDay.Ticks.
-        private const long MaxTimeTicks = 863_999_999_999;
+        // MaxTimeTicks is the max tick value for the time in the day.
+        private const long MaxTimeTicks = TimeSpan.TicksPerDay - 1;
 
         /// <summary>
         /// Represents the smallest possible value of TimeOnly.