DATEDIFF problem

[Hans Kanns]

Hello,
i am trying to figure out how soon employees will have birthday.
But

SELECT DATEDIFF( CONCAT( YEAR( CURRENT_DATE ) , "-", MONTH( birthday ) , "-", DAY( birthday) ) , CURRENT_DATE ) FROM employee;


gives out a different result on some tuples compared to

SELECT DAYOFYEAR( birthday ) - DAYOFYEAR( CURRENT_DATE ) FROM employee;

Some results differ by one day.
kindly help me figure out why and which SELECT-statement is the better one. Thanks!