function lastModified(uri) {
	return new Promise((resolve, reject) => {
		$.ajax({
			url: uri,
			type: "HEAD"
		}).done((d, s, xhr) => {
			let date = Date.parse(xhr.getResponseHeader("Last-Modified"))
			resolve(date)
		}).fail((xhr, s, err) => {
			reject(s)
		})
	})
}