Weird date calculation problem

I am having a problem I do not understand. I am trying to find the sunday 
before an employee's start date.
The following works beautifully for ALL employees except one and I cannot 
figure out why that one fails.


The employee in question has a start date of 2006-03-14 or 3351297600

CL-USER 17 > (setf EmployeeStartDate 3351297600)

CL-USER 18 > (decode-universal-time EmployeeStartDate)

0
0
0
14
3
2006
1
T
5

CL-USER 19 > (setf EmployeeStartDay (nth 6 (multiple-value-list (decode-
universal-time EmployeeStartDate))))
1

CL-USER 20 > (if (/= EmployeeStartDay 6)
    (setf Employeeweekbeginning (- EmployeeStartDate (* (+ 1 EmployeeStartDay) 
86400)))
    (setf Employeeweekbeginning EmployeeStartDate)
    )
3351124800

CL-USER 21 > (decode-universal-time Employeeweekbeginning)
0
0
23
11
3
2006
5
NIL
5

As you can see it returns the Saturday before instead of Sunday, it is off by 1 
hour.
Since 86400 = (* 24 60 60) or 1 day why would I be an hour off? Like I said ALL 
the other dates work just not this one. Am I missing something?

Thanks,

William