Error in formula -

While trying below FORMULA TRAILHEAD (https://developer.salesforce.com/trailhead/point_click_business_logic/formula_fields), getting error while saving FORMULA. Any help

Formula Object - CASE
FORMULA Return- number of days between the account’s Last Activity Date and today
Formula Code-

Account.LastModifiedDate  -    TODAY()

Error - Error: Incorrect parameter type for operator '-'. Expected Number, DateTime, received Date

If I change TODAY() to NOW() the compilation error gets resolved but challenge evaluation fails with following errors

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

Thanks
Priya