计算Cocoa中两个时间点之间的秒数，即使系统时钟在中途发生变化

I'm writing a Cocoa OS X (Leopard 10.5+) end-user program that's using timestamps to calculate statistics for how long something is being displayed on the screen. Time is calculated periodically while the program runs using a repeating NSTimer. I'm writing a Cocoa OS X (Leopard 10.5+) end-us