Lets say you are building your course and you have made everything visible to students and you haven't set any date restrictions. Students would be able to log on to the course before the start of the semester and view content. This might be ok for the syllabus, books, and contact information, but you may want to set the date restriction so that content and test could not be seen until the semester starts.
This particular screen is the one that is found when you add a folder. However whether you are setting the date restrictions for a quiz, a learning unit, or anything else the interface remains the same.
Because a folder can contain multiple items, if you want to restrict the view of several separate items, links, etc, you can drop them in one folder. You need to set the folder to be visible, because you will be using the date restriction to actually control the display of the folder. Visibility over-rides the date restriction. In other words you may have set date restrictions to display the folder after a specific date, but if visibility is set to no, the students will not see the folder.
You can set the dates using one of two methods. One is to use the pull down menu to set the month, day, year, and time. The second is to click on the calendar.
By clicking on the icon a small calendar opens and you can select your dates. You will still need to use the pull down menus to set the time.

One word of caution when using date restriction with a quiz. DO NOT set the date and time restriction to mirror the Set Time Limit of a quiz. If you set a time limit of 30 minutes for a quiz, do not restrict the date and time to reflect that same thirty minutes. Let the Set Time Limit function by setting the date restriction at a wider range. For example, students can take a test anytime from Friday afternoon to Monday morning (the test window), but they will only allowed 30 minutes to complete the test (actual test time).