In Linux, the current time is maintained by keeping the number of seconds elapsed since midnight of January 01, 1970 (called epoch); the second elements in each of these represent the time elapsed since the last second in microseconds and nanoseconds, respectively:

struct timespec {
        __kernel_time_t  tv_sec;                   /* seconds */
        long            tv_nsec;          /* nanoseconds */
};
#endif

struct timeval {
        __kernel_time_t          tv_sec;           /* seconds */
        __kernel_suseconds_t     tv_usec;  /* microseconds */
};

Time (counter value) read from the clock source needs to be accumulated and tracked somewhere; the structure struct tk_read_base, defined in include/linux/timekeeper_internal.h, serves this purpose:

struct tk_read_base {
        struct clocksource        *clock;
        cycle_t                  (*read)(struct clocksource *cs);
        cycle_t                  mask;
        cycle_t                  cycle_last;
        u32                      mult;
        u32                      shift;
        u64                      xtime_nsec;
        ktime_t                  base_mono;
};

The structure struct timekeeper, defined in include/linux/timekeeper_internal.h, keeps various timekeeping values. It's the primary data structure to maintain and manipulate the timekeeping data for different timelines, such as monotonic and raw:

struct timekeeper {
        struct tk_read_base       tkr;
        u64                      xtime_sec;
        unsigned long           ktime_sec;
        struct timespec64 wall_to_monotonic;
        ktime_t                  offs_real;
        ktime_t                  offs_boot;
        ktime_t                  offs_tai;
        s32                      tai_offset;
        ktime_t                  base_raw;
        struct timespec64 raw_time;

        /* The following members are for timekeeping internal use */
        cycle_t                  cycle_interval;
        u64                      xtime_interval;
        s64                      xtime_remainder;
        u32                      raw_interval;
        u64                      ntp_tick;
        /* Difference between accumulated time and NTP time in ntp
        * shifted nano seconds. */
        s64                      ntp_error;
        u32                      ntp_error_shift;
        u32                      ntp_err_mult;
};