How to properly set up snake case JSON for dotnet core api?

You forgot the quotes in:

"modified_on": 2017-02-08T19: 50: 31.690Z,

Changing this, works as well.

Since ASP.NET Core 3.0

services.AddMvc().AddNewtonsoftJson(options =>
{
    options.SerializerSettings.ContractResolver = new DefaultContractResolver
    {
        NamingStrategy = new SnakeCaseNamingStrategy()
    };
});