To make the calendar useful for us and for others who have heavily customized JIRA for their custom development processes we would want to be able to use a number of different custom dates on an issue to drive how/when it shows on the calendar. In our case we use a planned development start/end date set and a deployment date (start/end date would be the deployment date. Eventually we will have a requirements gathering start/end date as well. Some of our other groups have additional other start/end dates or just individual dates which would be used for both the start and end date.