dates in unix timestamp when exporting to Excel

When exporting the Gantt chart to Excel and setting the “column” properties then the “start_date” is converted to unix timestamp. Here’s an example

In this case you could add type property with “date” to columns with date.

		    columns: [
		        id: 'text',
		        header: 'Task'
		      }, {
		        id: 'start_date',
		        header: 'Start date',
                        type: 'date'
		      }, {
		        id: 'duration',
		        header: 'Duration',
                        type: 'number'