PostgreSQL: Get the TimeStamp of the beginning of a year, a month, or a day

To get the timestamp of the beginning of a year:

     //current year
      select date_trunc('YEAR', CURRENT_TIMESTAMP);

      //any year
      select date_trunc('YEAR', CURRENT_TIMESTAMP - INTERVAL '4 year');  or
      select date_trunc('YEAR', CURRENT_TIMESTAMP + INTERVAL '3 year'); or
      select TIMESTAMP '2019-01-01';

To get the timestamp of the beginning of a month:

      //current month
      select date_trunc('MONTH', CURRENT_TIMESTAMP);

      //any month
      select date_trunc('MONTH', CURRENT_TIMESTAMP - INTERVAL '9 month'); or
      date_trunc('MONTH', CURRENT_TIMESTAMP + INTERVAL '2 month'); or
      select TIMESTAMP '2019-08-01';

To get the timestamp of the beginning of a day:
   
      //current day
      select date_trunc('DAY', CURRENT_TIMESTAMP);

      //last day of the month
      select date_trunc('MONTH', CURRENT_DATE) + INTERVAL '1 month - 1 day';

      //last day of the month of a given date
      select date_trunc('MONTH', TIMESTAMP '2018-09-01) + INTERVAL '1 month - 1 day';

      //last day of the year
      select date_trunc('YEAR', CURRENT_DATE) + INTERVAL '1 year - 1 day';

      //last day of the year of a given date
      select date_trunc('YEAR', TIMESTAMP '2020-05-03') + INTERVAL '1 year - 1 day';

      //any day
      select date_trunc('DAY', CURRENT_TIMESTAMP - INTERVAL '12 day'); or
      select date_trunc('DAY', CURRENT_TIMESTAMP + INTERVAL '22 day'); or
      select TIMESTAMP '2019-08-25'; or
      select date_trunc('DAY', CURRENT_TIMESTAMP + INTERVAL '2 month + 5 day');
      select date_trunc('DAY', CURRENT_TIMESTAMP - INTERVAL '3 year - 2 month + 12 day');
   
Be aware when you minus an INTERVAL, the "- INTERVAL '3 year - 2 month - 12 day'" reduces three years from the current_timestamp, adds 2 months to it, and minuses twelve days from it.

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