Redmine

Redmine uses #xmlschema without arguments to format dates in XML output, which gives 0 decimals by default:

irb(main):002:0> Time.now.utc.xmlschema
=> "2015-03-14T09:04:32Z" 
irb(main):003:0> Time.now.utc.xmlschema(3)
=> "2015-03-14T09:04:41.221Z" 

source:/trunk/lib/redmine/views/builders/xml.rb

Note that I am using JSon instead of XML.

This is my Redmine 3.0.0 on the test server: http://76.126.10.142/redmine/issues/2.json
(use "test"/"test" credentials).

the payload it returns is

{
  "issue": {
    "id": 2,
    "project": {
      "id": 11,
      "name": "permanent test project" 
    },
    "tracker": {
      "id": 1,
      "name": "Bug" 
    },
    "status": {
      "id": 1,
      "name": "New" 
    },
    "priority": {
      "id": 2,
      "name": "Normal" 
    },
    "author": {
      "id": 1,
      "name": "Redmine Admin" 
    },
    "subject": "test123",
    "description": "",
    "done_ratio": 0,
    "spent_hours": 0.0,
    "custom_fields": [
      {
        "id": 1,
        "name": "my_custom_1" 
      },
      {
        "id": 2,
        "name": "custom_boolean_1",
        "value": "" 
      },
      {
        "id": 3,
        "name": "custom_multi_list",
        "multiple": true,
        "value": [
          "V1",
          "V3" 
        ]
      }
    ],
    "created_on": "2015-03-12T20:10:27.000Z",
    "updated_on": "2015-03-12T21:46:21.000Z" 
  }
}

server info:

Environment:
  Redmine version                3.0.0.stable
  Ruby version                   2.0.0-p598 (2014-11-13) [x86_64-linux]
  Rails version                  4.2.0
  Environment                    production
  Database adapter               Mysql2
SCM:
  Git                            1.9.1
  Filesystem                     
Redmine plugins:
  no plugin installed

even if you change Json format to match XML now, Redmine 3.0.0 version will still have this other format...
how can external clients detect the proper format? one way is to search for "." in the date string and thus detect that there are milliseconds there.
but a better (much more resilient) way would be for the server to provide the formatting string either in a separate REST endpoint ("give me all server settings please") or - even better - right inside the "issues/issue/all others" response:
"hey, you asked for issues, here is the result, and by the way, the date format I use is xxx-xxx-xxx-xxx-..."- this would make the response much easier to understand and client libraries (like Redmine Java API) - easier to maintain.