How do I get my iPhone app to perform functions depending on the time?

I want to learn how, for example, I could program a picture in Interface Builder to change bet开发者_开发知识库ween 3 pictures according to the time of day.

Or how I could program a certain UIButton to appear and disappear depending on the time of day—or even more specifically, the month of the year.

Where can i go to learn about this? And what is the formal name for this? Using time signatures? Or time stamp?

The NSDate class allows you to inspect the current time (also see Date and Time Programming Topics, and the NSTimer class allows you to schedule tasks.