GolamMostafa:
Library is good; it solves the problem in a minute. Someday, Adafruit or other respected individuals might discontinue the supplies of the Library Functions. Therefore, based on Arduino's basic functions (like Wire.beginTransmission(), Wire.write(), Wire.endTransmission(), Wire.requestFrom(), Wire.read()), we may try writing our own codes standing on one level lower platform. This way, we may slowly acquire the ability of writing Register Level instructions that might open opportunity to understand the 'System'.
Well, the RTC is a relatively easy piece of hardware to play with, however it requires getting used to understanding the registers and the datasheet thoroughly. Sometimes you have to do things in a particular order. Also to be complete you should have at least some conversion routines to and from epoch time. At least, if you are expecting to do complex calendar time calculations.
I keep and archive all the libraries I have downloaded and used! 