I had to add some code to set this for some reporting needs. This should be part of the offering ServiceNow provides.

Below is my business rule, and fix script to catch new records and fix

old records.

// Business Rule
// Table: Task [task]
// Active: true
// Advanced: true
// When: Before
// Order 100000000
// Insert: true
// Update:true
// Condition:
// calendar_durationISEMPTY^
// opened_atISNOTEMPTY^
// closed_atISNOTEMPTY^
// sys_class_name!=incident^
// sys_class_name!=problem
//
// Script
var opened = current.opened_at.getDisplayValue();
var closed = current.closed_at.getDisplayValue();
current.calendar_duration = gs.dateDiff(opened, closed, false);
current.business_duration = gs.calDateDiff(opened, closed, false);

Here's my fix script.

var log = 'calc. old durations';
var query = '';
query += 'calendar_durationISEMPTY^';
query += 'opened_atISNOTEMPTY^';
query += 'closed_atISNOTEMPTY^';
query += 'sys_class_name!=incident^';
query += 'sys_class_name!=problem^';

var task = new GlideRecord('task');
task.addEncodedQuery(query);
task.query();
var x = 0;
while(task.next()){
  x++;
  if(x%1000 === 0){
    gs.log('x: ' + x + ' of ' + task.getRowCount(), log);
  }
  var opened = task.opened_at.getDisplayValue();
  var closed = task.closed_at.getDisplayValue();
  task.business_duration = gs.calDateDiff(opened, closed, false);
  task.calendar_duration = gs.dateDiff(opened, closed, false);
  task.setWorkflow(false);
  task.autoSysFields(false);
  task.update();
}