Using 2 calculated attributes in a formula

I need some help to figure this out, please. I want to calculate the days between 2 dates that are derived from custom calculated attributes. 

My custom metric formula:

(DATE_DIFF([Custom Attribute 1 - Timestamp],[Custom Attribute 2 - Timestamp],"nb_of_hours")/60/24)

If I replace either one of the attributes in the formula above with a system date field like Ticket Created Date the calculation works, however with 2 custom attributes no results are returned. I am guessing it probably has something to do with needing to have each attribute perform its own calculation first and return a value. 

My attribute calculations:

Attribute 1: 

IF ([Changes - Field name]="custom field" 
AND [Changes - New value]="custom_field_value" 
 AND DATE_FIRST_FIX([Update - Timestamp],[Update ticket ID],[Changes - Field name],[Changes - New value])=[Update - Timestamp]
  )
  THEN [Update - Timestamp]

ENDIF

Attribute 2:

IF(
  [Changes - Field name]="group_id" 
  AND [Changes - New value]="xxxxx"
  AND DATE_FIRST_FIX([Update - Timestamp],[Update ticket ID],[Changes - Field name],[Changes - New value])=[Update - Timestamp]
  )
  THEN [Update - Timestamp]
  ENDIF