String substitution '%node_start_time' not working in email

I'm using CCK Date for my events. In the signup confirmation email I wrote, %user_name, %node_title and %node_url all get substituted properly, but %node_start_time does not. I was able to work around this with the Replacement Token [field_date-view] for my defined field.

My cck field is named field_date, with the label being 'Date'. I confirmed in my date content type under 'Signup settings' that I selected 'Date' as the 'Date field to user with signup' (the other two options being and ).

As I said, I have a workaround that works. But I'm curious why the substitution string doesn't work and hope the reason doesn't affect other functionality.