I need assistance implementing the Last Activity Time or Last Modified Time into custom forms.  Everything I've tried is not working.

I have ZohoCRM set up to sort my contacts based on Aged Importance.  If I don't need to contact the client, they don't show up on my list.  If I need to contact them, those who are more Important show up at the top of the list.  Less important clients move up the list the longer I delay contacting them.

In order for this to work, I need to be able to compare the current time with the Last Activity Time.  Days Since Last Contact is a custom formula using the code below.  The code does not show an issue with syntax, but it does not get updated with new records.  How do I calculate this?

Datecomp(Now(),${Contacts.Last Activity Time})/60/24  

 The procedure is a little complicated, so here is some additional detail if you need it.

1. Each Contact has a custom Importance field (0 is unimportant and 9 is highly important)
2. Each Contact has a custom Contact Frequency field... How often (in days) the person should be contacted (30, 60 90, etc.)
3. Each Contact has a custom Aged Importance field.

• Aged Importance is 0 if I don't need to contact them.  For example...it has been less than 60 days since I've contacted them and their Contact Frequency=60
• Aged Importance is equal to Importance if I need to contact them.  For example... it has been 75 days since I've contacted them and their Contact Frequency=60.  
• A contact is past due if it has been 30 days since I should have contacted them.  If Contact Frequency is 60 days, then this contact would be past due at 90 days.
• Aged Importance increases by 2 if the contact is 30 days past due.
• Aged Importance increases by 4 if the contact is 60 days past due.
• Aged Importance increases by 6 if the contact is 90 days past due.

On a side note:  why won’t formula fields update with existing records yet?!

Any help is appreciated.  I need this functionality and am nearly ready to switch to a different CRM if I can’t get this to work.