Adding created_at and modified_at columns in MySQL5.5 and below as DATETIME fields. These versions didn't allow default for any date/datetime column to be the value of a function such as NOW() or CURRENT_DATE. This can be alternatively achieved using triggers.

adding_default_values_for_date_fields.sql

-- Add the columns created_at & modified_at to your required table.
ALTER TABLE 
  table_name 
ADD 
  COLUMN created_at DATETIME NULL
ADD 
  COLUMN modified_at DATETIME NULL;

-- Initialize the data with the current timestamp.
UPDATE 
  TABLE table_name 
SET 
  created_at = NOW(), 
  modified_at = NOW() 
WHERE 
  1;

-- Create a trigger which ensures that when new rows are added, the default value is NOW().
CREATE TRIGGER table_name__before_insert BEFORE
INSERT
  ON table_name FOR EACH ROW 
SET 
  NEW.created_at = NOW(), 
  NEW.modified_at = NOW();

-- Create a similar trigger for updating rows.  
CREATE TRIGGER table_name__before_update BEFORE 
UPDATE 
  ON table_name FOR EACH ROW 
SET 
  NEW.created_at = OLD.created_at, 
  NEW.modified_at = NOW();