Rounding up causing abnormal/inconsistent results

[Meghan]

I need some help figuring out why my Excel timesheet formula is
occasionally returning abnormal results.
The formula calculates the total time (C1), then rounds the result up
to the nearest quarter hour (D1). (Formulas are below)
The formula works correctly most of the time, however, when I enter
certain time values, the results are not correct!
If I calculate time passed between 1:00 am and 2:00 am, the formula
returns the correct result: 1.00
However, if I calculate time passed between 1:00 pm and 2:00 pm, the
formula returns an incorrect result: 1.25
If I calculate time passed between 1:30 pm and 2:30 pm, the formula
returns the correct result: 1.00
I have done some testing, and found four time-spans that cause the
rounding error (below).
I am at a complete loss as how to explain the inconsistent results!
Can someone please help me? A thousand Thank You's!!!


(FORMULAS)
A1: hh:mm
B1: hh:mm
C1: =SUM((B1-A1)*24)
D1: =ROUNDUP((SUM(C1))/0.25,0)*0.25

(CORRECT RESULT)
A1: 1:00 am
B1: 2:00 am
C1: 1.00
D1: 1.00

(INCORRECT RESULT)
A1: 1:00 pm
B1: 2:00 pm
C1: 1.00
D1: 1.25

(Time-spans that cause inaccurate results)
1:00 pm - 2:00 pm
4:00 pm - 5:00 pm
7:00 pm - 8:00 pm
10:00 pm - 11:00 pm