• JsLint - a well known/used code quality tool for validating JSON.
• JSON Editor - (I haven't used this one yet, but it purports to provide a GUI-ish interface for editing JSON.)

(See .NET Development → Visual Studio → JSON & JSON Schema)

• JSLint plugin for Notepad++ - should also be available via the Plugin Manager, which is typically installed by default.
• jq is a lightweight and flexible command-line JSON processor. It can be used to query and transform JSON.
• JSON Web Token (JWT) - a compact URL-safe means of representing claims to be transferred between two parties.

• http://json-schema.org/ is the official home of JSON Schema.
• Understanding JSON Schema is a very useful online book for getting started.
• The Jasonary project also has some useful documentation.

• JsonSchema validation - Draft 4 is assumed. Provides:
  • Validation of data against your own schema
  • Schema syntax validation
  • Java ⇔ Json Schema
  • etc.
• JSON Validate - can provide schema, content and/or references for input.
• JSON Schema.net - validates JSON or JSON Schema, with form-based editing for blocks of JSON. Will also generate a Schema from JSON data.

JSON Schema does not support a datetime type directly. Instead, specify a string type with the format attribute:

{
    "type": "object",
    "properties": {
        "StartDate": {
            "type": "string",
            "format": "date-time"
        }
    }
}