为什么我的Date Picker脚本不起作用？

Question

我有这个脚本

function getYear() {
  var future = new Date();
  future.setDate(future.getDate() + 30);
  year = future.getFullYear();
  return this.year;
}

function getMonth() {
  var future = new Date();
  future.setDate(future.getDate() + 30);
  month = future.getFullMonth();
  return this.month;
}

function getDay() {
  var future = new Date();
  future.setDate(future.getDate() + 30);
  day = future.getFullDay();
  return this.day;
}

当我在这里打电话

{
  select_date prefix = 'time_expire_'
  id = '_time_expire'
  start_year = 'getYear()'
  start_month = 'getMonth()'
  start_day = 'getDay()'
  end_year = '+0'
  field_separator = ' / '
  field_order = 'MDY'
  default_all = true
}

不起作用。为什么？