Automatic updating of the first TIMESTAMP column occurs under any of the following conditions:  
 
The column is not specified explicitly in an INSERT or LOAD DATA INFILE statement.  
The column is not specified explicitly in an UPDATE statement and some other column changes value. (Note that an UPDATE that sets a column to the value it already has will not cause the TIMESTAMP column to be updated, because if you set a column to its current value, MySQL ignores the update for efficiency.)  
You explicitly set the TIMESTAMP column to NULL.  
TIMESTAMP columns other than the first may also be set to the current date and time. Just set the column to NULL or to NOW().  
 
You can set any TIMESTAMP column to a value different from the current date and time by setting it explicitly to the desired value.