06-17-2010
The exact name and location of "utmp" and "wtmp" files varies. See "man who" to find out the names on your system. Then check whether the files exist and have size, and that "wtmp" is not size 2Gb (i.e. too big). If the "utmp" and "wtmp" files are missing they have to be created manually with the correct permissions for your system prior to rebooting the system.
Your output from your "last reboot" just shows that there is no "reboot" record in the "wtmp" file possibly because that file has been deleted, nulled or corrupted since the computer was last booted. Many sites age this file weekly to stop it getting too big.
An error from "who -b" suggests that the "utmp" file has been deleted, nulled or corrupted since the computer was last booted and that it does not contain a "system boot" record.
The "uptime" command uses the same information as "who -b" with the advantage that it is correct when system has been up for more than a year.
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utmp(4) Kernel Interfaces Manual utmp(4)
NAME
utmp, wtmp - Login records
SYNOPSIS
#include <utmp.h>
DESCRIPTION
The utmp file records information about who is currently using the system. The file is a sequence of utmp entries, as defined in struct
utmp in the utmp.h file.
The utmp structure gives the name of the special file associated with the user's terminal, the user's login name, and the time of the login
in the form of time(3). The ut_type field is the type of entry, which can specify several symbolic constant values. The symbolic constants
are defined in the utmp.h file.
The wtmp file records all logins and logouts. A null user name indicates a logout on the associated terminal. A terminal referenced with
a tilde (~) indicates that the system was rebooted at the indicated time. The adjacent pair of entries with terminal names referenced by a
vertical bar (|) or a right brace (}) indicate the system-maintained time just before and just after a date command has changed the sys-
tem's time frame.
The wtmp file is maintained by login(1) and init(8). Neither of these programs creates the file, so, if it is removed, record keeping is
turned off. See ac(8) for information on the file.
FILES
RELATED INFORMATION
ac(8), init(8), last(8), lastcomm(8), login(1), who(1), wtmpconvert(8). delim off
utmp(4)