Cannot use dash in data structure

I have a structure like this.
# Data Structures
## UserData
- _id: 571a28adb94555081fa84671 (required, string) - Unique identifier
- email: email@example.com (required, string) - User email
- secret: notasecret (required, string) - User password
- name: User 1 (required, string) - User name
- created_at: 2016-04-22T13:35:41.600Z (required, string) - create time

But here is the result:
# Body

``` json
{
"_id": "571a28adb94555081fa84671",
"email": "email@example.com",
"secret": "notasecret",
"name": "User 1",
"created_at": "2016"
}
```
# Schema

``` json
{
"type": "object",
"properties": {
"_id": {
"type": "string",
"description": "Unique identifier"
},
"email": {
"type": "string",
"description": "User email"
},
"secret": {
"type": "string",
"description": "User password"
},
"name": {
"type": "string",
"description": "User name"
},
"created_at": {
"type": "string",
"description": "04-22T13:35:41.600Z (required, string) - create time"
}
},
"required": [
"_id",
"email",
"secret",
"name"
],
"$schema": "http://json-schema.org/draft-04/schema#"
}
```

You can see the `created_at` has some problem. I cannot add the string with dashes.