specifies the date period in the header of the lightbox
| start | Date | the date when a task is scheduled to begin |
| end | Date | the date when a task is scheduled to be completed |
| task | Task | the task object |
gantt.templates.task_time = function(start,end,task){
return gantt.templates.task_date(start)+" - "+gantt.templates.task_end_date(end);
};