In version 1.12 you will be able to achieve this by adding a custom dropdown in the toolbar (in previous versions it is only possible to add buttons there):
var tc = new TimeChart({
container: "chart",
data: [...],
toolbar: {
items: [
{
label: "Export",
align: "right",
cssClass: "DVSL-bar-btn-export",
dropDownItems: [
{
label: "Image (PNG)",
onClick: function () { tc.export("png"); }
},
{
label: "Excel (XLSX)",
onClick: function () { tc.export("xlsx"); }
}
]
}
]
}
});