System.Text.Json: 'The JSON value could not be converted to System.DateTime'

JsonException: The JSON value could not be converted to System.DateTime means the date string in your JSON is in a shape the default DateTime handling of System.Text.Json did not accept. The inner exception says so in plain words: The JSON value is not in a supported DateTime format. Either send the date in a format it reads, or register a small converter that parses the format you receive.

In the case reproduced here the API sent 06/11/2026 14:30 (day, month, year). A JsonConverter<DateTime> that calls DateTime.ParseExact with dd/MM/yyyy HH:mm, added to JsonSerializerOptions.Converters, made the same JSON deserialize.

The error

Unhandled exception. System.Text.Json.JsonException: The JSON value could not be converted to System.DateTime. Path: $.BookedAt | LineNumber: 0 | BytePositionInLine: 52.
 ---> System.FormatException: The JSON value is not in a supported DateTime format.
   at System.Text.Json.ThrowHelper.ThrowFormatException(DataType dataType)
   at System.Text.Json.Utf8JsonReader.GetDateTime()
...
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)
   at Program.<Main>$(String[] args) in C:\...\dt\Program.cs:line 17

Why it happens

The JSON was {"Name":"Maria Garcia","BookedAt":"06/11/2026 14:30"} and the target property was public DateTime BookedAt { get; set; }. The stack trace shows where it failed: Utf8JsonReader.GetDateTime() threw a FormatException, and the serializer wrapped it in a JsonException that names the property path ($.BookedAt) and the byte position. The Path part of the message is the useful clue: it tells you which property held the string the reader refused.

A string like 06/11/2026 is also ambiguous to a human reader: it can mean 6 November or 11 June. A converter that names the exact format removes the guess, and the output below shows which reading it chose.

The fix

var options = new JsonSerializerOptions();
options.Converters.Add(new DayMonthYearConverter());
var fixedBooking = JsonSerializer.Deserialize<Booking>(json, options)!;
Console.WriteLine($"{fixedBooking.Name} {fixedBooking.BookedAt:yyyy-MM-dd HH:mm}");
Console.WriteLine(JsonSerializer.Serialize(fixedBooking, options));
...
public class DayMonthYearConverter : JsonConverter<DateTime>
{
    private const string Format = "dd/MM/yyyy HH:mm";

    public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => DateTime.ParseExact(reader.GetString()!, Format, CultureInfo.InvariantCulture);

    public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializerOptions options)
        => writer.WriteStringValue(value.ToString(Format, CultureInfo.InvariantCulture));
}

Running dotnet run -- fix printed:

Maria Garcia 2026-11-06 14:30
{"Name":"Maria Garcia","BookedAt":"06/11/2026 14:30"}

The first output line shows the date read as 6 November 2026, the second shows the converter's Write method sending it back in the same shape, so a round trip with the same API keeps its format. Pass the same options instance to every call that reads or writes these bookings.

How it was reproduced

A console project from dotnet new console on .NET SDK 10.0.403 (net10.0, no extra packages; System.Text.Json is part of the framework), a Booking class with a DateTime property and JsonSerializer.Deserialize<Booking>(json) with default options. Run with dotnet run for the error and dotnet run -- fix for the converter.

Frequently asked

How do I deserialize a dd/MM/yyyy date with System.Text.Json?

Write a JsonConverter<DateTime> whose Read method calls DateTime.ParseExact with the format dd/MM/yyyy HH:mm and CultureInfo.InvariantCulture, then add it to JsonSerializerOptions.Converters and pass those options to Deserialize.

What does Path: $.BookedAt mean in a JsonException?

It is the JSON path of the value that failed. $ is the root object and BookedAt is the property, so the string in BookedAt is the one the serializer could not turn into a DateTime.

Why does the inner exception say not in a supported DateTime format?

The serializer read the date with Utf8JsonReader.GetDateTime, which threw a FormatException for the string 06/11/2026 14:30. The outer JsonException wraps that FormatException and adds the path and position.

More decoded errors in the Fixes category. The same message for a number sent as text is covered in The JSON value could not be converted to System.Int32.