"Tried to load unspecified class: Time" error when specifying timestamp without quotes in .gitlab-ci.yml

Summary

A line like:

released_at: 2021-03-15T08:00:00Z

in .gitlab-ci.yml causes a syntax error with the following message:

Tried to load unspecified class: Time

Steps to reproduce

1. Navigate to a project's CI Lint page (https://example.gitlab.com/user/project/-/ci/lint)

2. Enter the following content:

   create release:
     script:
       - echo "Creating a new release..."
     release:
       released_at: 2021-03-15T08:00:00Z
       tag_name: hello
       description: world
   

3. Click Validate

What is the current bug behavior?

Validation fails with the message Tried to load unspecified class: Time.

What is the expected correct behavior?

Validation should not fail.

Or, if we determine this is the expected behavior, we should document this here.

Output of checks

This bug happens on GitLab.com.

Workaround

The timestamp can be surrounded in quotes:

released_at: '2021-03-15T08:00:00Z'