Created Field:
Created Formula field "Days Since Last Update" to return Numeric value.
Used Formula "Account.LastActivityDate -  TODAY()" under advanced Formula
upon running it..its not updating this filed.
What I am missing in this?

I am working on the Set UP and Manage Shield Platform Encryption and have checked all components twice and I still get an error that Platform Encryption is not setup correctly on my org.  How can I be certain it is reading the correct DE org?  
 

A scenario on Trailhead:
To complete this challenge, add a validation rule which will block the insertion of a contact if the contact is related to an account and has a mailing postal code (which has the API Name MailingPostalCode) different from the account's shipping postal code (which has the API Name ShippingPostalCode).

Name the validation rule 'Contact must be in Account ZIP Code'.
A contact with a MailingPostalCode that has an account and does not match the associated Account ShippingPostalCode should return with a validation error and not be inserted.
The validation rule should ONLY apply to contact records with an associated account. Contact records with no associated parent account can be added with any MailingPostalCode value. (Hint: you can use the ISBLANK function for this check)

How to do it..as i tried multiple ways..i am surely missing on something. Please help