hwclock

Display the current time as reported by the hardware clock

$ sudo hwclock

$ sudo hwclock [[-w|--systohc]]

$ sudo hwclock [[-s|--hctosys]]

hwclock [FUNCTION] [OPTION...] where FUNCTION is --adjust, --get, --hctosys, --set, --systohc

--adjust
    Adjust hardware clock incrementally from drift file

--badyear
    Handle 2-digit years (19xx assumed before 2000)

--clock=
    Select clock: rtc (default), adjtimex, pmtimer

--date=
    Set hardware clock to specified date (with --set)

--debug
    Enable debugging output

--directisa
    Access RTC directly via ISA bus

--get
    Read and display hardware clock time

--hctosys
    Set system time from hardware clock

--help
    Display usage summary

--localtime
    Assume hardware clock is in local time (not UTC)

--noipv6
    Disable IPv6 support (for predictable)

--predict
    Predict hardware clock using delay and rate

--sethwclock
    Legacy: set system time from hardware clock

--set
    Set hardware clock from current system time or --date

--setdate=
    Set hardware clock date only (YYYY-MM-DD)

--show
    Synonym for --get

--systohc
    Set hardware clock from system time

--systz
    Update system timezone from /etc/localtime

--test
    Test mode: no changes made

--utc
    Assume hardware clock is in UTC (default)

--verbose
    Increase output verbosity

--version
    Display version information

hwclock is a utility to read or set the hardware clock, also known as the Real Time Clock (RTC), on IBM PC compatible systems. The RTC maintains time even when the system is powered off, powered by a small battery.

It enables synchronization between the RTC and the Linux system time (maintained by the kernel). Common workflows include:
• Setting system time from RTC on boot (--hctosys).
• Setting RTC from system time on shutdown (--systohc).
• Directly reading (--get) or setting (--set) the RTC.

By default, hwclock assumes the RTC stores time in UTC unless --localtime is specified. It supports adjustments for drift (--adjust) and direct hardware access (--directisa). Most operations require root privileges. On modern systems, timedatectl or systemd may handle this automatically.

Requires root for write operations. May not work on VMs without RTC emulation. Use --directisa only if CMOS access fails. Drift file (/etc/adjtime) must be writable. Inconsistent behavior on big-endian or non-x86 systems.

/etc/adjtime stores RTC drift rate for --adjust. Format: drift_in_seconds_per_day UTC/localtime 0.

-r for --get, -s for --hctosys, -w for --systohc are deprecated aliases.

Originally written by David Mosberger in 1993 as part of util-linux. Maintained in the util-linux project since 1995 by Karel Zak and others. Evolved to support new hardware interfaces and systemd integration.

date(1), timedatectl(1), rtc(4), rtcwake(8)