date_examples.rb

require 'json-schema'
require 'json'

schema = JSON.parse(
<<JSONSCHEMA
{
  "type": "object",
  "required": [
    "ceterms:foundingDate"
  ],
  "properties": {
    "ceterms:foundingDate": {
      "anyOf": [
        {
          "type": "string",
          "format": "date"
        },
        {
          "type": "string",
          "format": "date-time"
        }
      ]
    }
  }
}
JSONSCHEMA
)
puts schema.to_json

%w{
2017
2017-03
2017-03-31
2017-03-31T01:37:59Z
20170331T013759Z
2017-03-31T01:37:59+00:00
}.each do |date|
  data = { 'ceterms:foundingDate' => date }
  print data.to_json

  print ' valid? '
  puts JSON::Validator.validate(schema, data)
end

$ ruby date_examples.rb
{"type":"object","required":["ceterms:foundingDate"],"properties":{"ceterms:foundingDate":{"anyOf":[{"type":"string","format":"date"},{"type":"string","format":"date-time"}]}}}
{"ceterms:foundingDate":"2017"} valid? true
{"ceterms:foundingDate":"2017-03"} valid? true
{"ceterms:foundingDate":"2017-03-31"} valid? true
{"ceterms:foundingDate":"2017-03-31T01:37:59Z"} valid? true
{"ceterms:foundingDate":"20170331T013759Z"} valid? true
{"ceterms:foundingDate":"2017-03-31T01:37:59+00:00"} valid? true