Assuming you have WinCC V6 you can pass dynamic parameters to most objects in the report system. To do this you need to check the Tag boxes and assign a (usually internal) string tag. You then write the parameter value into the tag and it gets passed to the report.

For the alarm control you would need to check the Tag box in the Selection window and assign an internal text tag. In runtime you would write something like "DATETIME >= '2006-06-20 08:00:00' AND DATETIME <= '2006-06-20 09:00:00'" to the string tag. This will then be passed to the report which will then show you all alarms between 8am and 9am on the 20th June 2006 [:)]

You can find out more about filtering the alarm control in the WinCC manual.
A good trick is to play with the alarm control in runtime. Click on the Selection toolbar button, enter a start and end time. Click OK. Click on the Selection button again. At the bottom of the screen it shows you the filter string that you have just asked for.

I hope this makes sense. If not, just ask.