HWCLOCK(8) System Administration HWCLOCK(8) (NAME) hwclock - (RTC) (SYNOPSIS) hwclock [function] [option...] (DESCRIPTION) hwclock . : (Hardware Clock) (drift) NTP (epoch) ( Alpha) . 2.26 --hctosys --directisa --update-drift . . (FUNCTIONS) . --show . -a, --adjust . (The Adjust Function) . --getepoch; --setepoch Alpha RTC . (epoch) . (Epoch) . BIOS . --setepoch --epoch . : hwclock --setepoch --epoch=1952 RTC . hwclock Alpha . ISA . --param-get=parameter; --param-set=parameter=value RTC. RTC (Backup Switchover Mode) RTC . parameter RTC ( include/uapi/linux/rtc.h ) (alias). --help . parameter value 0x . --param-index number RTC number. number 0 . --predict --date /etc/adjtime. ( alarm) . rtcwake(8) . hwclock ( << >> ) . -r, --show; --get ISO 8601. UTC . --localtime . . --get /etc/adjtime . hwclock ( << >> ) . -s, --hctosys . . (The Adjust Function) . UTC . UTC . --hctosys /etc/adjtime --localtime --utc . : (DST) . UTC (LOCAL vs UTC) . --hctosys . TZ /etc/localtime tzset(3) . tz_dsttime . ( settimeofday(2) .) --hctosys settimeofday(2) << >> NTP persistent_clock_is_local . (reboot) . (Automatic Hardware Clock Synchronization by the Kernel) . (mount) / . . . ( << >> NTP) --hctosys . /etc/adjtime . --update-drift --systohc ( ) . --noadjfile --hctosys . /etc/adjtime . hwclock UTC . . hwclock --localtime --adjust . hwclock . . hwclock . --set --date /etc/adjtime. --update-drift () . --set . --update-drift . --systz --hctosys . 2.6 . --hctosys : o UTC . hwclock . o << >> NTP . o . settimeofday(2) . . . -w, --systohc /etc/adjtime. --update-drift () . --systohc . --update-drift . --vl-read, --vl-clear RTC . --vl-read (Voltage Low) . --vl-clear RTC . include/uapi/linux/rtc.h . RTC . -h, --help . -V, --version . (OPTIONS) --adjfile=filename /etc/adjtime . --date=date_string --set --predict . hwclock --set --date='16:45' hwclock --predict --date='2525-08-14 07:11:05' UTC . --localtime . . "+5 minutes" hwclock Enter . . . --delay=seconds . 0.5 ( ) rtc_cmos RTC 0 . RTC ( sysfs) 0.5 . MC146818A ( x86) . . . . . -D, --debug --verbose. --debug . --directisa ISA x86 x86_64 . . hwclock I/O . hwclock rtc RTC . 2.26 rtc directisa . . --rtc . --epoch=year --setepoch . year . (ULONG_MAX - 1). -f, --rtc=filename rtc hwclock . : /dev/rtc /dev/rtc0 /dev/misc/rtc. IA-64: /dev/efirtc /dev/misc/efirtc -l, --localtime; -u, --utc . UTC . --localtime --utc hwclock . ( ) . --utc --localtime (--set --systohc --adjust) /etc/adjtime . adjtime UTC . : (DST) . UTC (LOCAL vs UTC) . --noadjfile /etc/adjtime . hwclock . --utc --localtime . --test /etc/adjtime (--verbose ). --update-drift /etc/adjtime . --set --systohc . . . . 2.26 hwclock --systohc () : o NTP << >> . o <<>> (cold) . <<>> . . o . . o ( 2.31 --update-drift RTC ). hwclock /etc/adjtime . . . (The Adjust Function) . . . . . . -v, --verbose hwclock . (NOTES) (Clocks in a Linux System) : (The Hardware Clock): ( ) . ISA ISA . . (hardware clock) (real time clock) RTC (BIOS clock) CMOS . Hardware Clock hwclock . (persistent clock) . ISA . I2C SPI . (The System Clock): . ( ISA ISA .) . 00:00:00 UTC ( UTC). . . . DOS ISA . date(1) . . : hwclock --systohc . hwclock . -- . ( ) : TZ /etc/localtime tzset(3) . . vfat . vfat . << >> NTP . / persistent_clock_is_local << >> . (Automatic Hardware Clock Synchronization by the Kernel) . hwclock --hctosys --systz TZ /etc/localtime . : ) tz_minuteswest ( DST) UTC ) tz_dsttime (DST) . . settimeofday(2) . (Hardware Clock Access Methods) hwclock . I/O rtc rtc . rtc udev . rtc --rtc . rtc . . ISA hwclock I/O 0x70 0x71 << CMOS>> . I/O (superuser) . --directisa . I/O . hwclock ISA rtc . (The Adjust Function) . - . (systematic drift) . --adjust hwclock . : hwclock /etc/adjtime . adjtime . adjtime . hwclock --set . hwclock adjtime . hwclock --set --update-drift . hwclock adjtime . hwclock --adjust . hwclock adjtime . . . hwclock --adjust . hwclock : adjtime . --update-drift --set --systohc () adjtime . /etc/adjtime . --adjust . --adjust . hwclock --hctosys adjtime . --adjust . adjtime . --adjust . Adjtime (The Adjtime File) hwclock . adjtime (ASCII) : : : ) ) UTC ) ( clock(8)) . : : UTC . ( ) . . : "UTC" "LOCAL". (UTC) . hwclock . adjtime clock(8) hwclock . (Automatic Hardware Clock Synchronization by the Kernel) . . . NTP . (NTP . RFC 1305 .) << >> . ( 0x0040 ) time_status (unset) . 'status' adjtimex --print ntptime . NTP << >> . hwclock --hctosys . NTP << >> . << >> --hctosys --systz . . UTC . . persistent_clock_is_local . --hctosys --systz adjtime . . hwclock --adjust << >> NTP . ISA (ISA Hardware Clock Century value) CMOS ISA () . hwclock << >> . CMOS hwclock . "direct ISA" . ACPI . (DATE-TIME CONFIGURATION) (Keeping Time without External Synchronization) : o ( NTP cron job) . o . 'RIGHT' (POSIX vs 'RIGHT') . o : adjtimex --tick value --frequency value hwclock --hctosys o : hwclock --systohc o adjtimex ntptime . NTP . . <<>> . . . . . . hwclock ( ) . --tick --frequency adjtimex(8) . : tick frequency . ( adjtimex ntptime -f ppm .) adjtimex . . . . --log adjtimex . sntp date -Ins . tick frequency . adjtimex(2) . . . / . . . : 1 NTP . 2 System Clock ( ) ! 3 . 4 . 5 . 6 hwclock --update-drift . : --systohc ( 6a). hwclock /etc/adjtime . . sntp date -Ins . UTC (LOCAL vs UTC) (DST) : o . o . ISA . hwclock <<>> . UTC . hwclock --date . POSIX UTC . UTC . . . RealTimeIsUniversal UTC . 'RIGHT' (POSIX vs 'RIGHT') tzset(3) . 'right' (Time Zone Database) tz zoneinfo . zoneinfo : posix 'right'. 'Right' ( zoneinfo-leaps ) (leap seconds) posix . 'right' (UTC + leap seconds) (TAI - 10) . . 'right' (civil time) UTC . : . /usr/share/zoneinfo . posix 'right' : TZ='right/Europe/Dublin'. zoneinfo posix 'right' zoneinfo : /usr/share/zoneinfo /usr/share/zoneinfo-posix /usr/share/zoneinfo-leaps . 'right' . zoneinfo . World Clock South_Pole MTA hwclock UTC /usr/share/zoneinfo . . . zoneinfo posix 'right' TZDIR . (EXIT STATUS) : EXIT_SUCCESS ('0' ) . EXIT_FAILURE ('1' ) (syntax) . (ENVIRONMENT) TZ . TZDIR . (FILES) /etc/adjtime hwclock. adjtime_config(5) . /etc/localtime . /usr/share/zoneinfo/ . hwclock : /dev/rtc /dev/rtc0 /dev/misc/rtc /dev/efirtc /dev/misc/efirtc (SEE ALSO) date(1), adjtime_config(5), adjtimex(8), gettimeofday(2), settimeofday(2), crontab(1p), tzset(3) (AUTHORS) Bryan Henderson clock(8) Charles Hedrick Rob Hooft Harald Koenig. . (REPORTING BUGS) . . (AVAILABILITY) hwclock util-linux . . util-linux 2.42.3 2026-09-02 HWCLOCK(8)