UPDATE RLT005
SET CtrDatBaixa = '2018-12-15 00:00:00'
WHERE
CtrDatBaixa BETWEEN '2018-12-15 00:00:00' AND '2018-12-15 23:59:59'

UPDATE RLT005
SET CtrDatBaixa = '2018-12-31 00:00:00'
WHERE
CtrDatBaixa BETWEEN '2018-12-31 00:00:00' AND '2018-12-31 23:59:59'

UPDATE RLT005
    SET CtrDatBaixa = '2018-12-15 00:00:00'
    WHERE CtrDatBaixa BETWEEN '2018-12-15 00:00:00' AND '2018-12-15 23:59:59' ||
          CtrDatBaixa BETWEEN '2018-12-31 00:00:00' AND '2018-12-31 23:59:59'