Originally Posted by xelawho
and you can't provide that option?
this may seem niggling, but to me it speaks to a serious usability problem on the web. If you need to explain how to use your page with a whole separate set of instructions (which you would in this case, or else have to explain why the user's choice is not validating, or else just leave it to them to figure out for themselves) then you've already lost. Nobody reads instructions. They check boxes, click buttons, see what happens and if it's not what they expect, maybe they investigate why not.
My idea is that it's your job as designer to make the page as intuitive as possible - it's not the user's job to try and figure out what it was that you were thinking when you made the page.
I can provide both options such as 08:00 AM - 09:00 AM and 08:30 AM - 09:30 AM but if a user selects both options then 08:30 is a duplicate time here.