Conditional Statement By Time–Possible?

Is there a way that I can serve:

Box A (during the first 30 minutes of every hour)
Box B (during the last 30 minutes of every hour)

Or

Box A (during the first 15 minutes of every hour)
Box B (during the last 45 minutes of every hour)

etc.

Or is there a plugin someone can think of that will manage this?

Thanks for any help ??