SQL delete problem on session table

[buducom]

I have the following line:
mysql_query("DELETE FROM sessionstable WHERE (TO_DAYS(now()) - TO_DAYS(lasthit)) > 1 ")or DIE(mysql_error());

sometimes i surf around on my website where many people are logged in at the same time.

and my session gets deleted. even though lasthit is not smaller than 1 day.

lasthit is a datetime field.

any idea?
this is the only DELETE query in the whole script on the sessionstable.

[Weedpacket]

Does this happen at any particular time of day?

[buducom]

no . seems to be happening at various intervals
when i throw out the line completely it has no problem
The thing is i dont want the sessionstable to get too big