Dateオブジェクトを生成すると時刻まで設定されますが日付比較ができないなど時間部分を削除したいことがあると思います。

getメソッド

date1.gas

function dateTest1() {
  var today = new Date();
  today = new Date(today.getFullYear(), today.getMonth(), today.getDate());

  console.log(today);
}

console

11:47:20 AM	Notice	Execution started
11:47:20 AM	Info	Sat Jan 04 2025 00:00:00 GMT+0900 (Japan Standard Time)
11:47:21 AM	Notice	Execution completed

setメソッド

date2.gas

function dateTest2() {
  var today = new Date();  
  today.setHours(0);
  today.setMinutes(0);
  today.setSeconds(0);

  console.log(today);
}

console

11:50:02 AM	Notice	Execution started
11:50:02 AM	Info	Sat Jan 04 2025 00:00:00 GMT+0900 (Japan Standard Time)
11:50:03 AM	Notice	Execution completed

toDateStringメソッド

toDateStringメソッドで日付文字列を取得して時間情報を付加する

date3.gas

function dateTest3() {
  var today = new Date();
  console.log(today.toDateString());
  today = new Date(today.toDateString() + " 00:00:00 GMT+0900 (Japan Standard Time)");
}

console

12:06:27 PM	Notice	Execution started
12:06:27 PM	Info	Sat Jan 04 2025
12:06:27 PM	Info	Sat Jan 04 2025 00:00:00 GMT+0900 (Japan Standard Time)
12:06:29 PM	Notice	Execution completed

getTimeメソッド

getTimeメソッドで1970年1月1日からの経過ミリ秒を取得して
86400000ミリ秒(1日)で剰余算して時間に当たるミリ秒を引き
32400000ミリ秒(9時間)で日本時間(UTC+9)を引く

date4.gas

function dateTest4(){
  var today = new Date();
  // 現在日時-時分秒-9時間(日本はUTC+9)
  today = new Date(today.getTime()-(today.getTime()%86400000)-32400000);

  console.log(today);
}

console

11:52:10 AM	Notice	Execution started
11:52:11 AM	Info	Sat Jan 04 2025 00:00:00 GMT+0900 (Japan Standard Time)
11:52:11 AM	Notice	Execution completed