Hello - I've completed the challenge for Using Formula Fields in Trailhead and I can see, by creating a new Case, that my formula is working correctly. However, when I click the Check Challenge button, I get the 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"

My formula is:  TODAY() -  Last_Activity_Date__c







Can you please help me figure out how to get past this? Thanks.