Is there a known issue with this Challenge or am I doing something wrong?

The Challenge:
Your support team has asked for improved visibility on account activity level at the time they’re helping with customer issues. Specifically, when they’re looking at a case, they’d like to see an at-a-glance view of the number of days since the case’s related account was last active. Create the formula using these requirements.

• The formula should be on the Case object.
• The formula should be of return type Number.
• The formula should be named 'Days Since Last Update' and have a resulting API Name of 'Days_Since_Last_Update__c'.
• The formula should return the number of days between the account’s Last Activity Date and today.

Here is what I did:
Setup > Customize > Cases > Fields > Clicked "New" under 'Case Custom Fields & Relationships'
Field Label: Days Since Last Update
Field Name: Days_Since_Last_Update [Why does this field not reflect "Days_Since_Last_Update__c" as is requested in the challenge?]
From the 'Formula Return Type' picklist I selected "Number"
From the 'Decimal Places' picklist I selected "0"
The formula is "Account.LastActivityDate - TODAY ()"
Check Syntax = No errors
Under 'Blank Field Handling' I selected "Treat blank fields as zeroes"

Now when I navigate to a Case Record...the "Days Since Last Update" field is blank.
When I Check the Challenge it says "Challenge Not yet complete... here's what's wrong: 
The 'Days_Since_Last_Update__c' formula field did not return the correct number of days between an Account’s Last Activity Date and today"

I cannot figure out what I am doing wrong.
Any guidance?