Passing UTC DateTime to Web API HttpGet Method results in local time

The query string parameter value you are sending 2014-04-01T00:00:00Z is UTC time. So, the same gets translated to a time based on your local clock and if you call ToUniversalTime(), it gets converted back to UTC.

So, what exactly is the question? If the question is why is this happening if sent in as query string but not when posted in request body, the answer to that question is that ASP.NET Web API binds the URI path, query string, etc using model binding and the body using parameter binding. For latter, it uses a media formatter. If you send JSON, the JSON media formatter is used and it is based on JSON.NET.

Since you have specified DateTimeZoneHandling.Utc, it uses that setting and you get the date time kind you want. BTW, if you change this setting to DateTimeZoneHandling.Local, then you will see the same behavior as model binding.

I ended up just using the ToUniversalTime() method as parameters come in.

If you want the conversion to be transparent, then you could use a custom TypeConverter:

public sealed class UtcDateTimeConverter : DateTimeConverter
{
    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        return ((DateTime)base.ConvertFrom(context, culture, value)).ToUniversalTime();
    }
}

and wire it up using:

TypeDescriptor.AddAttributes(typeof(DateTime), new TypeConverterAttribute(typeof(UtcDateTimeConverter)));

Then the query string parameter will be instantiated as DateTimeKind.Utc.

So, for those of you who do not wish to override string-to-date conversion in your entire application, and also don't want to have to remember to modify every method that takes a date parameter, here's how you do it for a Web API project.

Ultimately, the general instructions come from here:

https://docs.microsoft.com/en-us/aspnet/web-api/overview/formats-and-model-binding/parameter-binding-in-aspnet-web-api#model-binders

Here's the specialized instructions for this case:

1. In your "WebApiConfig" class, add the following:

       var provider = new SimpleModelBinderProvider(typeof(DateTime),new UtcDateTimeModelBinder());
       config.Services.Insert(typeof(ModelBinderProvider), 0, provider);
   

2. Create a new class called UtcDateTimeModelBinder:

   public class UtcDateTimeModelBinder : IModelBinder
   {
       public bool BindModel(HttpActionContext actionContext,
           ModelBindingContext bindingContext)
       {
           if (bindingContext.ModelType != typeof(DateTime)) return false;
   
           var val = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
           if (val == null)
           {
               return false;
           }
   
           var key = val.RawValue as string;
           if (key == null)
           {
               bindingContext.ModelState.AddModelError(bindingContext.ModelName,
                   "Wrong value type");
               return false;
           }
   
           DateTime result;
           if (DateTime.TryParse(key, out result))
           {
               bindingContext.Model = result.ToUniversalTime();
               return true;
           }
   
           bindingContext.ModelState.AddModelError(bindingContext.ModelName,
               "Cannot convert value to Utc DateTime");
           return false;
       }
   }