That will work - but it will have to do a calculation for every value of YOURCOL. You may want to change it around to:

SELECT * FROM YOURTABLE WHERE YOURCOL < UNIX_TIMESTAMP() - 24 * 3600;
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That way the db only does one calculation and can then compare it to the values of YOURCOL and even still use an index if there is one.

my @a=qw(random brilliant braindead); print $a[rand(@a)];