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 https://jsfiddle.net/egomaksab/a9L62svh/
In this case you could add type property with “date” to columns with date.
E.g.
gantt.exportToExcel({
columns: [
{
id: 'text',
header: 'Task'
}, {
id: 'start_date',
header: 'Start date',
type: 'date'
}, {
id: 'duration',
header: 'Duration',
type: 'number'
}
]
});