JamesNK/Newtonsoft.Json · error · JsonSerializationException

Expected date object value.

Error message

Expected date object value.

What it means

Thrown by JavaScriptDateTimeConverter.WriteJson when the value is not a DateTime or DateTimeOffset. This converter serializes dates as a JavaScript Date constructor call (new Date(ticks)); only DateTime and DateTimeOffset are accepted, so any other type hits the else branch.

Solutions

1. Ensure only DateTime or DateTimeOffset values reach JavaScriptDateTimeConverter.
2. Remove or replace the converter on properties whose type changed.
3. For non-DateTime types, write a custom converter that converts to DateTime first.

Example fix

// before: property changed type, converter left in place
[JsonConverter(typeof(JavaScriptDateTimeConverter))]
public long TimestampMs { get; set; }

// after: remove converter from non-date type
[JsonProperty("timestampMs")]
public long TimestampMs { get; set; }

object value = GetValue();
if (value != null && !(value is DateTime) && !(value is DateTimeOffset))
    throw new InvalidOperationException($"JavaScriptDateTimeConverter cannot serialize {value.GetType()}");

static bool IsDateType(object v) =>
    v is DateTime || v is DateTimeOffset;

try { conv.WriteJson(writer, value, serializer); }
catch (JsonSerializationException ex) when (ex.Message.Contains("Expected date object value"))
{
    throw new InvalidOperationException($"JavaScriptDateTimeConverter got {value?.GetType()}", ex);
}

When it happens

Trigger: Applying JavaScriptDateTimeConverter to a property whose runtime value is not DateTime/DateTimeOffset. On builds without DateTimeOffset support, passing a DateTimeOffset also fails. Passing a nullable DateTime whose Value was boxed as something else.

Common situations: Switching a property type from DateTime to string/int but keeping the JavaScriptDateTimeConverter attribute. Serializing DateOnly/TimeOnly with this converter. Polymorphic values whose runtime type differs from the declared type.

Related errors

• Expected date object value.
• Unexpected value when converting date. Expected DateTime or…
• Cannot convert date value that is before Unix epoch of…
• Converter cannot write specified value to JSON.
• Expected Version object value

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/b03903132257a853. Report an issue: GitHub.

Appendix: source

Thrown at Src/Newtonsoft.Json/Converters/JavaScriptDateTimeConverter.cs:61

        public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
        {
            long ticks;

            if (value is DateTime dateTime)
            {
                DateTime utcDateTime = dateTime.ToUniversalTime();
                ticks = DateTimeUtils.ConvertDateTimeToJavaScriptTicks(utcDateTime);
            }
#if HAVE_DATE_TIME_OFFSET
            else if (value is DateTimeOffset dateTimeOffset)
            {
                DateTimeOffset utcDateTimeOffset = dateTimeOffset.ToUniversalTime();
                ticks = DateTimeUtils.ConvertDateTimeToJavaScriptTicks(utcDateTimeOffset.UtcDateTime);
            }
#endif
            else
            {
                throw new JsonSerializationException("Expected date object value.");
            }

            writer.WriteStartConstructor("Date");
            writer.WriteValue(ticks);
            writer.WriteEndConstructor();
        }

        /// <summary>
        /// Reads the JSON representation of the object.
        /// </summary>
        /// <param name="reader">The <see cref="JsonReader"/> to read from.</param>
        /// <param name="objectType">Type of the object.</param>
        /// <param name="existingValue">The existing property value of the JSON that is being converted.</param>
        /// <param name="serializer">The calling serializer.</param>
        /// <returns>The object value.</returns>
        public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
        {
            if (reader.TokenType == JsonToken.Null)

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