Yes, you can always edit the hand-in and peer grading date and time. Find the assignment title on the sidebar and click on it to open the assignment overview. Click on the calendar in the upper right corner of the date and time box, this will allow you to change the date and time.
If the assignment is already open, you can not change the deadline.
You can allow students to submit hand-ins late by going to the assignment page, and then selecting the Hand-ins tab. Scrolling down the page there is a list of students that have not handed in the assignment. To allow for late hand-in for all students click Allow all to hand in late or you can select which students hand-in late by clicking Allow next to the students name.