Basic scrollbar question

[david at anon.nu]

I have this scroll bar - horizontal, scale style, with showValue off

I am using it as a control to allow scrolling from a start date to an 
end date (dates in seconds). So I set the start and endValues.

Next I set the lineInc to 24*60*60 which jumps a day - ok

But I have to set the pageInc as follows to: 8*24*60*60 ??? Why not 7?

Also not sure how to control the marks - it would make sense if they 
were representing the pageInc value - but it is a smaller setting.

Any help here - not used a scale scroolbar before in detail?