Symptoms

In the scenario: escalation email to recipient's manager after 72 hours and then again every 24 hours until activity is completed, escalations are not correctly calculating working hours.

Using the Escalate On rule with a Dynamic Date/Time = Add Hours(EventInstanceStartDate, 27) And Then After 9 hours to be repeated 99 times causes the escalation to fire 36 clock hours after the event start date, and then repeat every 9 working hours.
 

Diagnoses

The Add Hours function does not add working hours, it only adds clock hours, so you need to add the actual number of hours, not the number of working hours.

The "And then after" is an extension, adding the configured number of hours to the Dynamic Time/Date field.

In this scenario, adding the 9 hour "And then after" extension to the 27 hours in the Add Hours function causes the escalation to fire after 36 clock hours.
 

Resolution

In order to configure the escalation to fire 72 hours from the event start date, and then repeat every 9 working hours, a possibility was noted to configure the Dynamic DateTime field using Add Hours(EventInstanceStartDate, 63), with the "And then after" extension configured to add 9 hours, repeating 99 times. This sent the escalation after 72 hours, and then repeated every 9 working hours.

However, this configuration does not take weekends into account and so in some situations the escalation will fire prior to the full three working days.

A feature request has been logged for the escalations to take working hours into account.