Subsecond Resolution In Rails

23 Jul 2013 by Parker Selbert

The de-facto standard for representing dates and times is ISO8601. The standard describes a variety of date/time formats, but as developers on the modern web the format that we routinely encounter looks like this:

YYYY-MM-DDThh:mm:ssTZD (eg 1997-07-16T19:20:30+01:00)

If you read slightly farther in the spec you’ll see that there is also an enhanced form that includes fractions of a second:

YYYY-MM-DDThh:mm:ss.sTZD (eg 1997-07-16T19:20:30.45+01:00)

Note the decimal near the end of the second format (ss.s). The decimal allows sub-second precision within date/time, which is potentially useful. Ruby’s standard Date and Time libraries support parsing the higher resolution format via methods like Date.iso6801. Though they support parsing they don’t output fractions of a second when the #iso8601 method is called on an instance of Time: #=> "2013-07-22T20:57:21-05:00"

Changes in Rails 4

Those of you with a test suite that verifies timestamps may have noticed a change when upgrading to Rails 4. Given a request spec similar to this:

it 'lists an existing post' do
  post = Post.create(title: 'Strawberries')

  get "{}"

  decoded = JSON.parse(last_response.body)

  expect(decoded).to eq(
    'id'         =>,
    'title'      => 'Strawberries',
    'created_at' => post.created_at.iso8601,
    'updated_at' => post.updated_at.iso8601

The spec will fail when comparing both timestamps, even though they have been formatted as iso8601. The fake diff below attempts to illustrate this:


The problem stems from an inconsistent overriding of as_json that only applies to instances of TimeWithZone but does not effect the as_json monkey patching of Time, Date, and DateTime.

Temporary Solution

The sub-second resolution change, as small as it is, was enough to break timestamp parsing within some iOS apps. To prevent backward incompatibilities I’ve laid my own monkey patch over TimeWithZone:

# config/initializers/time_with_zone.rb

module ActiveSupport
  class TimeWithZone
    def as_json(options = nil)

Going forward I’m looking to expose configuration that allows the resolution to be configured. The default behavior in Rails 4 will remain 3 digits of resolution, but setting the resolution to 0 will remove it entirely.