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  12 .\" Copyright 2018 Gary Mills
  13 .\" Copyright (c) 2003, Sun Microsystems, Inc. All Rights Reserved.
  14 .\"
  15 .Dd January 31, 2018
  16 .Dt RTC 1M
  17 .Os
  18 .Sh NAME
  19 .Nm rtc
  20 .Nd provide all real-time clock and UTC-lag management
  21 .Sh SYNOPSIS
  22 .Nm
  23 .Op Fl csuvw
  24 .Op Fl z Ar zone-name
  25 .Sh DESCRIPTION
  26 The Real Time Clock (RTC) is the hardware device on x86 computers that maintains
  27 the date and time.
  28 The RTC is battery-powered, so that it keeps running when the computer is shut
  29 down.
  30 It can be set from the BIOS and also from the operating system running on the
  31 computer.
  32 The RTC has no setting for the time zone or for Daylight Saving Time (DST).
  33 It relies on the operating system for these facilities and for automatic changes
  34 between standard time and DST.
  35 .Pp
  36 On x86 systems, the
  37 .Nm
  38 command reconciles the difference in the way that time is established between
  39 UNIX and Windows systems.
  40 The internal clock on UNIX systems utilizes Universal Coordinated Time (UTC)
  41 while Windows systems usually expect the RTC to run in local time, including DST
  42 changes.
  43 .Pp
  44 Without arguments,
  45 .Nm
  46 displays the currently configured time zone string for the RTC.
  47 The currently configured time zone string is based on what was last recorded by
  48 .Nm Fl z Ar zone-name .
  49 .Pp
  50 The
  51 .Nm
  52 command is not normally run from a shell prompt; it is generally invoked by the
  53 system.
  54 Commands such as
  55 .Xr date 1
  56 and
  57 .Xr rdate 1M ,
  58 which are used to set the time on a system, invoke
  59 .Nm Fl c
  60 to ensure that daylight savings time (DST) is corrected for properly.
  61 .Sh OPTIONS
  62 .Bl -tag -width Ds
  63 .It Fl c
  64 This option checks for DST and makes corrections to the RTC if necessary.
  65 It is normally run once a day by a
  66 .Xr cron 1M
  67 job.
  68 .Pp
  69 If there is no RTC time zone or
  70 .Pa /etc/rtc_config
  71 file, this option will do nothing.
  72 .It Fl s
  73 This option specifies that the RTC runs in local standard time all year round.
  74 It is incompatible with Windows, but is convenient if only one operating system
  75 is to be run on the computer.
  76 The
  77 .Xr cron 1M
  78 command is not necessary, and should not be run.
  79 .It Fl u
  80 This option specifies that the RTC runs in UTC time.
  81 As a side effect, it sets the time zone in
  82 .Pa /etc/rtc_config
  83 to UTC.
  84 Windows can operate in UTC time, but requires a registry change to do so.
  85 The
  86 .Xr cron 1M
  87 command is not necessary.
  88 .It Fl v
  89 This option specifies that the RTC tracks local time, including DST changes.
  90 This is the default.
  91 It accomodates Windows with no changes.
  92 The
  93 .Xr cron 1M
  94 command is necessary to change the RTC when DST is in effect.
  95 .It Fl w
  96 This option does nothing.
  97 It is present for compatibility with Solaris 11.
  98 .It Fl z Ar zone-name
  99 This option, which is normally run by the system at software installation time,
 100 is used to specify the time zone in which the RTC is to be maintained.
 101 It updates the configuration file
 102 .Pa /etc/rtc_config
 103 with the name of the specified zone and the current UTC lag for that zone.
 104 If there is an existing
 105 .Pa /etc/rtc_config
 106 file, this command will update it.
 107 If not, this command will create it.
 108 .El
 109 .Sh FILES
 110 .Bl -tag -width "/etc/rtc_config"
 111 .It Pa /etc/rtc_config
 112 The data file used to record the time zone and UTC lag.
 113 This file is completely managed by
 114 .Nm .
 115 At boot time, the kernel reads the UTC lag from this file, and uses it to set
 116 the system time.
 117 .El
 118 .Sh ARCHITECTURE
 119 .Sy x86
 120 .Sh SEE ALSO
 121 .Xr date 1 ,
 122 .Xr cron 1M ,
 123 .Xr rdate 1M ,
 124 .Xr attributes 5