title: Postgres, Update TIMESTAMP to current date but preserve time of day
body: <p>In my Postgres database, I have the following table:</p>
  
  <pre><code>SELECT start_at, end_at FROM schedules;
  
  +---------------------+---------------------+
  | start_at            | end_at              |
  |---------------------+---------------------|
  | 2016-09-05 16:30:00 | 2016-09-05 17:30:00 |
  | 2016-09-05 17:30:00 | 2016-09-05 18:30:00 |
  | 2017-08-13 03:00:00 | 2017-08-13 07:00:00 |
  | 2017-08-13 03:00:00 | 2017-08-13 07:00:00 |
  | 2017-08-13 18:42:26 | 2017-08-13 21:30:46 |
  | 2017-08-10 00:00:00 | 2017-08-10 03:30:00 |
  | 2017-08-09 18:00:00 | 2017-08-10 03:00:00 |
  | 2017-08-06 23:00:00 | 2017-08-07 03:00:00 |
  | 2017-08-07 01:00:00 | 2017-08-07 03:48:20 |
  | 2017-08-07 01:00:00 | 2017-08-07 03:48:20 |
  | 2017-08-07 18:05:00 | 2017-08-07 20:53:20 |
  | 2017-08-07 14:00:00 | 2017-08-08 01:00:00 |
  | 2017-08-07 18:00:00 | 2017-08-07 20:48:20 |
  | 2017-08-08 08:00:00 | 2017-08-09 00:00:00 |
  | 2017-08-09 21:30:00 | 2017-08-10 00:18:20 |
  | 2017-08-13 03:53:26 | 2017-08-13 06:41:46 |
  +---------------------+---------------------+
  </code></pre>
  
  <p>Assume I also have an ID column, what I want to do is update all the start and end times to be for today (now), what is the most efficient SQL to accomplish this? My table could have millions of rows.</p>