I was hoping to get the same output like I get with

SELECT seq, TIMESTAMPDIFF(SECOND,NOW(),(NOW() + INTERVAL seq SECOND)) df FROM seq_1_to_60

your "timestamp - timestamp" does not do what you think it does. Try:

SELECT seq, (NOW() + INTERVAL seq second) - NOW() df, now() + interval seq second, now(), cast(now() + interval seq second as unsigned), cast(now() as unsigned) FROM seq_1_to_60

you can see that the second column is the difference between values in two last columns.

Well thank you for explanation. Anyway the numeric expression of datetime subtraction is still non-deterministic, therefore it's useless. For several samples I decided that this is number of seconds between. But that is not true. So I had to rewrite my views queries to use timestampdiff function. And I guess I'm not the last person to get into this trap.