Serializing DateOnly and TimeOnly types with System.Text.Json

.NET 6 introduced 2 new types to work with dates and times in .NET. One is the DateOnly type that represents the Date portion of a DateTime. The other is the TimeOnly type that represents the Time portion of a DateTime.

Unfortunately when you try to use these types inside your data contracts, you get into trouble when you try to Serialize them through the System.Text.Json.JsonSerializer.

	var dateOnly = DateOnly.FromDateTime(DateTime.Now);
	var timeOnly = TimeOnly.FromDateTime(DateTime.Now);
	var example = new { dateOnly, timeOnly };
	//This call will fail
	var json=System.Text.Json.JsonSerializer.Serialize(example);

view raw SerializeDateOnly.cs hosted with ❤ by GitHub

To solve this problem, we can create 2 custom JsonConverter types:

	public class DateOnlyConverter : JsonConverter<DateOnly>
	{
	private const string DateFormat = "yyyy-MM-dd";
	public override DateOnly Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
	{
	return DateOnly.ParseExact(reader.GetString(), DateFormat, CultureInfo.InvariantCulture);
	}
	public override void Write(Utf8JsonWriter writer, DateOnly value, JsonSerializerOptions options)
	{
	writer.WriteStringValue(value.ToString(DateFormat, CultureInfo.InvariantCulture));
	}
	}

view raw DateOnlyConverter.cs hosted with ❤ by GitHub

	public class TimeOnlyConverter : JsonConverter<TimeOnly>
	{
	private const string TimeFormat = "HH:mm:ss.FFFFFFF";
	public override TimeOnly Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
	{
	return TimeOnly.ParseExact(reader.GetString(), TimeFormat, CultureInfo.InvariantCulture);
	}
	public override void Write(Utf8JsonWriter writer, TimeOnly value, JsonSerializerOptions options)
	{
	writer.WriteStringValue(value.ToString(TimeFormat, CultureInfo.InvariantCulture));
	}
	}

view raw TimeOnlyConverter.cs hosted with ❤ by GitHub

To use these converters, we need to create and pass an instance of the JsonSerializerOptions where we register these 2 converters:

	var dateOnly = DateOnly.FromDateTime(DateTime.Now);
	var timeOnly = TimeOnly.FromDateTime(DateTime.Now);
	var example = new { dateOnly, timeOnly };
	var options = new JsonSerializerOptions();
	options.Converters.Add(new DateOnlyConverter());
	options.Converters.Add(new TimeOnlyConverter());
	var json =JsonSerializer.Serialize(example,options);

view raw SerializeUsingConverters.cs hosted with ❤ by GitHub

The generated JSON looks like this:

	{
	"dateOnly":"2022-07-12",
	"timeOnly":"15:14:02.28045"
	}

view raw Example.json hosted with ❤ by GitHub

Hope that helps!