Controlling Animation with CAAnimation - iOS Programming: The Big Nerd Ranch Guide - page 641

Let's make the duration of this animation one second. That's enough time for the user to
see the spin but not so much time that they get bored waiting for it to complete.
Lastly, we need the values of the two keyframes: the fromValue and the toValue .
The documentation says that the transform.rotation is in radians, so that's how
we'll pass these values. A little geometry research tells us that no rotation is 0 radians and
a full rotation is 2 * PI radians. When using a CABasicAnimation , if you do not
supply a fromValue , the animation assumes that fromValue is the current value of
that property. The default value of the transform property is the identity matrix - no
rotation. This means you only have to supply the final keyframe to this animation object.
In TimeViewController.h , declare a new method.
- (void)spinTimeLabel;
In TimeViewController.m , edit the method showCurrentTime: to call this
method.
- (IBAction)showCurrentTime:(id)sender
{
NSDate *now = [NSDate date];
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setTimeStyle:NSDateFormatterMediumStyle];
[timeLabel setText:[formatter stringFromDate:now]];