There is a shift in thinking between boards like Arduino UNO and boards like Galileo, BeagleBone and R-PI.
'Traditional' arduino boards have direct access to hardware resources such as IO, ADC, RTC and so on. In the Galileo, and others, there is an operating system in place.
The operating system abstracts the hardware and users code cannot access hardware directly. (This is even true with GPIO on Galileo - your code is not directly setting registers on output ports in the processor, but is rather passing information to a kernel driver which access the hardware).
So, in order to access hardware features, such as RTC, you need to understand how the Operating System makes them available to you.
Galileo uses Linux as it's operating system and it has a rich and power set of features for accessing real time, elapsed time and for timing intervals in code.
There is a fabulous book on Linux, which is extremely useful: The Linux Programming Interface
Although not specific to Galileo it gives a great insight into how Linux works, and covers subjects such as timing in great depth.
If you are doing some serious work on Galileo, I would highly recommend ditching the Arduino IDE and getting to grips with one of the Linux distributions with Dev Tools included.
You can then write native code directly targeting Linux, on Galileo.