Use result of IF statement directly in same Statement

My Statement looks as follows:

SELECT *,
IF (valid_till = "9999-12-31 23:59:59", NOW(), valid_till) as valid_till, 
(valid_till - valid_from) as summary
FROM tablename

So the output of valid till looks correct, but the value of summary is still calculated by the value of "9999-12-31 23:59:59"

How can I achieve that summary is calculated directly by the valid_till value generated by the IF statement?

Thank you