Convert unix time to 12:00 AM of that same day

[Bryan Cantwell]

I'd asked before how to convert a unix timestamp to the hour that it is 
in (and got the perfect answer) :
1298999201 = 3/1/2011 11:06:41 AM
(1298999201 - (1298999201 % 3600)) = 3/1/2011 11:00:00 AM

Now getting the timestamp converted to midnight of that same day isn't 
as simple as:
1298999201 - (1298999201 % 85400)
That just gives me a unix time from yesterday...

How can I convert  1298999201 (3/1/2011 11:06:41 AM)  to 1298959200 
(3/1/2011 12:00:00 AM)?