在ASP.NET Core 8 Web API中，可以使用自定义转换器来解决将JSON值转换为System.DateOnly的问题。下面是一个示例代码：

首先，创建一个自定义转换器类，继承自JsonConverter类，并重写ReadJson和WriteJson方法：

using System;
using System.Buffers;
using System.Text.Json;
using System.Text.Json.Serialization;

public class DateOnlyConverter : JsonConverter
{
    public override DateOnly Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        string dateStr = reader.GetString();
        if (DateTime.TryParse(dateStr, out DateTime date))
        {
            return new DateOnly(date);
        }
        else
        {
            throw new JsonException("Invalid date format.");
        }
    }

    public override void Write(Utf8JsonWriter writer, DateOnly value, JsonSerializerOptions options)
    {
        writer.WriteStringValue(value.ToString());
    }
}

然后，在Startup.cs文件的ConfigureServices方法中注册自定义转换器：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System.Text.Json.Serialization;

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers()
            .AddJsonOptions(options =>
            {
                options.JsonSerializerOptions.Converters.Add(new DateOnlyConverter());
                options.JsonSerializerOptions.IgnoreNullValues = true;
                options.JsonSerializerOptions.PropertyNameCaseInsensitive = true;
                options.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
            });
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseRouting();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

现在，您可以在您的Web API控制器中使用System.DateOnly类型的参数，而不会出现转换错误：

using Microsoft.AspNetCore.Mvc;
using System;

[ApiController]
[Route("api/[controller]")]
public class DateController : ControllerBase
{
    [HttpPost]
    public IActionResult Post([FromBody] DateOnly date)
    {
        // 处理DateOnly类型的日期
        return Ok(date);
    }
}

这样，当您将带有日期值的JSON请求发送到该控制器的Post动作时，ASP.NET Core 8 Web API将能够将JSON值正确转换为System.DateOnly类型。