fix date time display (#2983)

* fix date time display

* Use ISO8601, fix nullability, improve validation

* Should probably override all of them to be consistent

* use local time format here

Co-authored-by: James Groom <OSSYoshiRulz+GitHub@gmail.com>

src/BizHawk.Common/BizDateTimeConverter.cs

@@ -0,0 +1,15 @@
+using System;
+using System.ComponentModel;
+using System.Globalization;
+
+namespace BizHawk.Common
+{
+	public class BizDateTimeConverter : DateTimeConverter
+	{
+		public override object? ConvertTo(ITypeDescriptorContext? context, CultureInfo? culture, object? value, Type destinationType)
+		{
+			if (value is not DateTime d || destinationType != typeof(string)) throw new NotSupportedException("can only do DateTime --> string");
+			return d.ToString();
+		}
+	}
+}

src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBA/MGBAHawk.ISettable.cs

@@ -163,6 +163,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBA
 			[DisplayName("RTC Initial Time")]
 			[Description("The initial time of emulation.  Only relevant when a game has an RTC chip and \"RTC Use Real Time\" is false.")]
 			[DefaultValue(typeof(DateTime), "2010-01-01")]
+			[TypeConverter(typeof(BizDateTimeConverter))]
 			public DateTime RTCInitialTime { get; set; }
 
 			[DisplayName("Save Type")]

src/BizHawk.Emulation.Cores/Consoles/Nintendo/NDS/MelonDS.ISettable.cs

@@ -91,6 +91,7 @@ namespace BizHawk.Emulation.Cores.Consoles.Nintendo.NDS
 			[DisplayName("Initial Time")]
 			[Description("Initial time of emulation.")]
 			[DefaultValue(typeof(DateTime), "2000-01-01")]
+			[TypeConverter(typeof(BizDateTimeConverter))]
 			public DateTime InitialTime
 			{
 				get => _initaltime;

src/BizHawk.Emulation.Cores/Consoles/WonderSwan/WonderSwan.ISettable.cs

@@ -88,10 +88,12 @@ namespace BizHawk.Emulation.Cores.WonderSwan
 			[DisplayName("Initial Time")]
 			[Description("Initial time of emulation.  Only relevant when UseRealTime is false.")]
 			[DefaultValue(typeof(DateTime), "2010-01-01")]
+			[TypeConverter(typeof(BizDateTimeConverter))]
 			public DateTime InitialTime { get; set; }
 
 			[Description("Your birthdate.  Stored in EEPROM and used by some games.")]
 			[DefaultValue(typeof(DateTime), "1968-05-13")]
+			[TypeConverter(typeof(BizDateTimeConverter))]
 			public DateTime BirthDate { get; set; }
 
 			[Description("True to emulate a color system.")]