Dynamically adjust fixture value in ruby tests

I'm testing my Rails application and a specific part of the project looks at how long ago a user was created. If I write a fixture for this, the only way I can think of right now is to define multiple users with different created_at values.

Is there a way to dynamically define the created_at value per test? I'd like to just define one user and each test will pass in a different value for created_at. If this is not possible, is there a better way I can test this?