I tried to install RTClib using the library manager of IDE, but it kept showing "in progress" forever. So, I searched for it online, downloaded it and put it in the libraries folder.
A few minutes ago, I deleted that downloaded folder and retried with the library manager. Again, I can always see "in progress" and it never ends. However, I restarted the IDE this time and compiled the code successfully.
It looks that the library manager had installed it correctly but didn't provide the right end, so I thought that it failed in the first time.
Thanks for your help!