Time is always something interesting to measure and to deal with, especially in embedded software that is, obviously, our main purpose here. The Arduino core includes several time functions that I'm going to talk about right now.
There is also a nice library that is smartly named SimpleTimer Library
and designed as a GNU LGPL 2.1 + library by Marcello Romani. This is a good library based on the millis() core function which means the maximum resolution is 1 ms. This will be more than enough for 99 percent of your future projects. Marcello even made a special version of the library for this book, based on micros().
The Arduino core library now also includes a native function that is able to have a resolution of 8 microseconds, which means you can measure time delta of 1/8,000,000 of a second; quite precise, isn't it?
I'll also describe a higher resolution library FlexiTimer2 in the last chapter of the book. It will provide a high-resolution, customizable timer.