Hi,
The trailhead challenge for Admin Advanced  Advanced Formulas (Level Up with Advanced Formulas) states:
Create a formula that shows where an Opportunity is in the pipeline.
Create a formula field that classifies an Opportunity as either “Early”, “Middle”, or “Late”. This formula field should use TODAY() to calculate what percentage of the time between an opportunity’s CreatedDate and CloseDate has passed, and label the opportunity accordingly.

My question is where is the createdDate field in the oppotunities object? The challenge implies this is an existing field but I cannot find it. 
Also what is meant by percentage of time passed in this context? Does this mean 'the time passed between the createdDate and closedDate' in relation to (i.e. divided by) 'the time passed between the createdDate and TODAY' ? 

Thanks in advance.