Date format issues with sumproduct formula?

Hi guys,

I've got a data table with a whole load of dates in the following format:

Column A: DD-MM-YYYY HH:MM:SS

I'm trying to do a sumproduct based on anything within a specific hour, so I've used the following formula to set up another Date column that ignores the MM:SS element of the date:

Column R =IF(DAY(B2)<10,"0"&DAY(B2)&"/"&MONTH(B2)&"/"&YEAR(B2)&" "&HOUR(B2)&":00",DAY(B2)&"/"&MONTH(B2)&"/"&YEAR(B2)&" "&HOUR(B2)&":00")

So in R2 I have 04/10/2012 10:00

On another sheet I've tried to set up a cumulative list of dates, increasing by 1 hour so that I can then use a sumproduct function to return certain values:

Cell A2 =Sheet1!R2 This returns 04/10/2012 10:00

I've then got a formula in cells A2 etc that adds 1 hour to the previous cell:

=A2+ TIME(1,0,0)

Now the sumproduct works for the A2 reference but not for A3 etc. I can see that there's a formatting difference between A2 and A3, as the number in A3 moved to the right ever so slightly. eg

A2 04/10/2012 10:00
A3 _04/10/2012 11:00
A4 _04/10/2012 12:00 <edit> the post isn't showing the slight indent so I've forced it across with a "_", ignore this
etc.

Any ideas what's going wrong? Am I going in completely the wrong direction? I'm sure it's some sort of formatting issue...

Cheers,

Jed.

Attach a sample file with examples and the required output for getting quick solution.

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Sixthsense