function getKey() {
  var filename = Utilities.formatDate(new Date(), 'GMT', 'yyyy-MM-dd');
  var files = DriveApp.getFilesByName(filename);
  if (files.hasNext()) {
    var ssId = SpreadsheetApp.open(files.next()).getId();
    SpreadsheetApp.getActiveSheet().getRange('A1').setValue(ssId);
  }
}