1 RTC(1M) Maintenance Commands RTC(1M) 2 3 NAME 4 rtc - provide all real-time clock and UTC-lag management 5 6 SYNOPSIS 7 rtc [-csuvw] [-z zone-name] 8 9 DESCRIPTION 10 The Real Time Clock (RTC) is the hardware device on x86 computers that 11 maintains the date and time. The RTC is battery-powered, so that it 12 keeps running when the computer is shut down. It can be set from the 13 BIOS and also from the operating system running on the computer. The RTC 14 has no setting for the time zone or for Daylight Saving Time (DST). It 15 relies on the operating system for these facilities and for automatic 16 changes between standard time and DST. 17 18 On x86 systems, the rtc command reconciles the difference in the way that 19 time is established between UNIX and Windows systems. The internal clock 20 on UNIX systems utilizes Universal Coordinated Time (UTC) while Windows 21 systems usually expect the RTC to run in local time, including DST 22 changes. 23 24 Without arguments, rtc displays the currently configured time zone string 25 for the RTC. The currently configured time zone string is based on what 26 was last recorded by rtc -z zone-name. 27 28 The rtc command is not normally run from a shell prompt; it is generally 29 invoked by the system. Commands such as date(1) and rdate(1M), which are 30 used to set the time on a system, invoke rtc -c to ensure that daylight 31 savings time (DST) is corrected for properly. 32 33 OPTIONS 34 -c This option checks for DST and makes corrections to the RTC if 35 necessary. It is normally run once a day by a cron(1M) job. 36 37 If there is no RTC time zone or /etc/rtc_config file, this option 38 will do nothing. 39 40 -s This option specifies that the RTC runs in local standard time 41 all year round. It is incompatible with Windows, but is 42 convenient if only one operating system is to be run on the 43 computer. The cron(1M) command is not necessary, and should not 44 be run. 45 46 -u This option specifies that the RTC runs in UTC time. As a side 47 effect, it sets the time zone in /etc/rtc_config to UTC. Windows 48 can operate in UTC time, but requires a registry change to do so. 49 The cron(1M) command is not necessary. 50 51 -v This option specifies that the RTC tracks local time, including 52 DST changes. This is the default. It accomodates Windows with 53 no changes. The cron(1M) command is necessary to change the RTC 54 when DST is in effect. 55 56 -w This option does nothing. It is present for compatibility with 57 Solaris 11. 58 59 -z zone-name 60 This option, which is normally run by the system at software 61 installation time, is used to specify the time zone in which the 62 RTC is to be maintained. It updates the configuration file 63 /etc/rtc_config with the name of the specified zone and the 64 current UTC lag for that zone. If there is an existing 65 /etc/rtc_config file, this command will update it. If not, this 66 command will create it. 67 68 FILES 69 /etc/rtc_config The data file used to record the time zone and UTC lag. 70 This file is completely managed by rtc. At boot time, 71 the kernel reads the UTC lag from this file, and uses it 72 to set the system time. 73 74 ARCHITECTURE 75 x86 76 77 SEE ALSO 78 date(1), cron(1M), rdate(1M), attributes(5) 79 80 illumos January 31, 2018 illumos