Trailhead Module 3:  Create a formula field that determines the number of days between today and the last activity date for a case's account.
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.

I created the following:

I created a new custom formula field with return type number and called Days Since Last Update.
I created this formula: Account.LastActivityDate - TODAY()

I get this error message:

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

Can you please assist what I did wrong?  I'm not sure if my formula is even correct with the challenge.  Please help!

Thank you.