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rtc(1m) [freebsd man page]

rtc(1M) 						  System Administration Commands						   rtc(1M)

rtc - provide all real-time clock and GMT-lag management SYNOPSIS
/usr/sbin/rtc [-c] [-z zone-name] DESCRIPTION
On x86 systems, the rtc command reconciles the difference in the way that time is established between UNIX and MS-DOS systems. UNIX systems utilize Greenwich Mean Time (GMT), while MS-DOS systems utilize local time. Without arguments, rtc displays the currently configured time zone string. The currently configured time zone string is based on what was last recorded by rtc-z zone-name. The rtc command is not normally run from a shell prompt; it is generally invoked by the system. Commands such as date(1) and rdate(1M), which are used to set the time on a system, invoke /usr/sbin/rtc -c to ensure that daylight savings time (DST) is corrected for properly. OPTIONS
-c This option checks for DST and makes corrections if necessary. It is normally run once a day by a cron job. If there is no RTC time zone or /etc/rtc_config file, this option will do nothing. -z zone-name This option, which is normally run by the system at software installation time, is used to specify the time zone in which the RTC is to be maintained. It updates the configuration file /etc/rtc_config with the name of the specified zone and the current GMT lag for that zone. If there is an existing rtc_config file, this command will update it. If not, this command will create it. FILES
/etc/rtc_config The data file used to record the time zone and GMT lag. This file is completely managed by /usr/sbin/rtc, and it is read by the kernel. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Architecture |x86 | +-----------------------------+-----------------------------+ |Availability |SUNWcsu | +-----------------------------+-----------------------------+ SEE ALSO
date(1), rdate(1M), attributes(5) SunOS 5.10 3 Oct 2003 rtc(1M)

Check Out this Related Man Page

TZ(5)								File Formats Manual							     TZ(5)

TZ - Time zone environment variable SYNOPSIS
TZ=zone[-]offset[dst[offset][,start[/time],end[/time]]] DESCRIPTION
The TZ environment variable tells functions such as the ctime(3) family and programs like date what the time zone and daylight saving rule is. The value of TZ has the POSIX standardized form shown in the synopsis. This form specifies the zone names, offsets from GMT, and day- light savings changeover times for at least the current year. zone A three or more letter name for the time zone in normal (winter) time. [-]offset A signed time telling the offset of the time zone westwards from Greenwich. The time has the form hh[:mm[:ss]] with a one of two digit hour, and optional two digit minutes and seconds. dst The name of the time zone when daylight savings is in effect. It may be followed by an offset telling how big the clock correction is other than the default of 1 hour. start/time,end/time Specifies the start and end of the daylight savings period. The start and end fields indicate on what day the changeover occurs. They must be in one of the following formats: Jn The Julian day n (1 <= n <= 365) ignoring leap days, i.e. there is no February 29. n The zero-based Julian day (0 <= n <= 365). Leap days are not ignored. Mm.n.d This indicates month m, the n-th occurrence of day d (1 <= m <= 12, 1 <= n <= 5, 0 <= d <= 6, 0=Sunday). The 5-th occurrence means the last occurrence of that day in a month. So M4.1.0 is the first Sunday in April, M9.5.0 is the last Sunday in Septem- ber. The time field indicates the time the changeover occurs on the given day. EXAMPLES
Greenwich Mean Time: TZ=GMT0 Middle European Time, 1 hour east from Greenwich, daylight savings starts on the last Sunday in March at 2 AM and ends on the last Sunday in October at 3 AM: TZ='MET-1MET DST,M3.5.0/2,M10.5.0/3' British time, daylight savings starts and ends at the same moment as MET, but in an earlier time zone: TZ=GMT0BST,M3.5.0/1,M10.5.0/2 The eastern european time zones also have the changeovers at the same absolute time as British time and MET. U.S. Eastern Standard Time, 5 hours west from Greenwich, daylight savings starts on the first Sunday in April at 2 AM and ends on the last Sunday in October at 2 AM: TZ=EST5EDT,M4.1.0/2,M10.5.0/2 It shouldn't surprise you that daylight savings in New Zealand is observed in the months opposite from the previous examples. It starts on the first Sunday in October at 2 AM and ends on the third Sunday in March at 3 AM: TZ=NZST-12NZDT,M10.1.0/2,M3.3.0/3 SEE ALSO
readclock(8), date(1). BUGS
You may have noticed that many fields are optional. Do no omit them, because the defaults are bogus. If you need daylight savings then fully specify the changeovers. West is negative, east is positive, ask any sailor. AUTHOR
Kees J. Bot ( TZ(5)
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