Why can I not use the START_DATE context in Global Absence Plan Use Rate formula type?

Summary:

I want to use the start date of the absence in a formula of type Global Absence Plan Use Rate. I am using the context START_DATE but this is not working. How can I determine the start date from this formula?

Content (required):

Formula is attached to Sickness absence plan and we are trying to select the Rate to use based on the length of the absence. we are using the number of days between EFFECTIVE_DATE context and START_DATE context in order to select a different Rate at 60 days into the absence. The EFFECTIVE_DATE context is working but the START_DATE context is not being set.