Assigning deadlines by time and not date

Can we assign a deadline by time ( say by 12pm) and not by date? Also can we assign time to a certain task by time ( say from 1am to 4am ) rather than 4 hrs on 12/5/07?

I am working on a project which is time based and the deadlines are on a given day at 8am. So i have to back track and schedule tasks from 2-5 am etc.. to meet the 8am deadline.

Please help!

Thanks in advance.