Confusion about the cancellation process.

Confusion about the cancellation process. -- Linux general technology-Linux programming and kernel information. The following is a detailed description. An article on the internet is as follows:
At login, the login program fills in such a structure, writes it to the utmp file, and adds it to the wtmp file. During logout, The init process erases the corresponding records in the utmp file (each byte is set to 0) and adds a new record to the wtmp file. Read the logout record in the wtmp file, and the ut_name field is cleared to 0. When the system is restarted, and before and after the system time and date are changed, special record items are added to the wtmp file.
I have two questions:
1. Since the ut_name field of the cancellation record in the wtmp file is cleared to 0 during cancellation, how can we know who logged out? Why clear the ut_name field?
2. When a user logs out, the user writes a cancellation record to the wtmp file. Does this record how long the user has logged on?
Thanks