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?

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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.