Linux datetime

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This article covers date and time issues on Linux including epoch time, timezones, hardware and system clock sync, ntpdate, and NTP.

Epoch time

UNIX Epoch Time is the number of seconds since 00:00:00 1970-01-01 UTC. This is useful for datetime stamps or generating unique names.

The epoch time can be retrieved using the `date` command:

$ date "+%s"
1266033437

Unfortunately, it is not trivial to convert epoch times to a human-readable format in a portable way. The following will work on most Linux systems and is probably the most portable:

$ EPOCH=`date "+%s"`
$ echo $EPOCH
1266033437
$ date +"%F %T %z" -d "1970-01-01 UTC + $EPOCH seconds"
2010-02-12 19:57:17 -0800

The `date` command in versions of GNU coreutils since 5.3.0 support the @ feature:

$ EPOCH=`date "+%s"`
$ echo $EPOCH
1266033437
$ date +"%F %T %z" -d @$EPOCH
2010-02-12 19:57:17 -0800

If you have GNU Awk installed then you can use the following command (Ubuntu/Debian systems do not have the 'gawk' package installed by default):

$ EPOCH=`date "+%s"`
$ echo $EPOCH
1266033437
$ echo $EPOCH | awk '{print strftime("%F %T %z",$1)}'
2010-02-12 19:57:17 -0800

You can use a Python one-liner to handle epoch times.

$ python -c "import time; print time.time()"
1396490714.62
$ python -c "import time, sys; print time.strftime('%Y-%m-%dT%H:%M:%S',time.localtime(float(sys.argv[1])))" 1396494023
2014-04-02T20:00:23

Supposedly, the following will work on BSD systems; unfortunately, not Mac OS X. And it is not portable with the GNU `date` command.

$ EPOCH=`date "+%s"`
$ echo $EPOCH
1266033437
$ date +"%F %T %z" -r $EPOCH
2010-02-12 19:57:17 -0800

This has been tested on Mac OS X and Linux. It may not work on very old versions of OS X.

# Pass an epoch time and this will return the date in human readable format.
# This works on Linux and OS X.
epoch_to_rfc_3339 ()
{
        EPOCH=$(echo $@ | sed -n "s/.*\([0-9]\{10\}\).*/\1/p");
        if ! date "+%FT%T:::%z" -d "1970-01-01 UTC ${EPOCH} seconds" 2>/dev/null; then
                date -r ${EPOCH} "+%FT%T:::%z"
        fi
}

date calculations with the Linux `date` command

The 'seconds' keyword may be written as 'seconds', 'second', 'secs', and 'sec'.

The `date` command can format a not just the current date, but any given date with the '-d' option. You can also express calendrical calculations from a given date or from the now time.

# date; date -d "- 1 week"
Fri May 28 13:43:36 PDT 2010
Fri May 21 13:43:36 PDT 2010

On Linux, the file '/proc/uptime' will give you the number of seconds the system has been running. From there you can use `date` to figure out what date the system booted. Of course, you can get the boot time using `who -b` or `last -x`, but that doesn't teach us anything.

date +"%F %T %z" -d "now UTC - $(cat /proc/uptime | cut -f 1 -d ' ' ) seconds"
2010-05-14 09:31:05 -0700

Current time minus a number of seconds

If you run `cat /proc/uptime`

# uptime
 13:35:57 up 14 days,  4:27,  4 users,  load average: 0.00, 0.01, 0.04

So, what date was 14 days, 4 hours, and 27 minutes ago?

# date -d "-14 days -4 hours -27 minutes"
Fri May 14 09:11:19 PDT 2010

Bash function to return formatted date in days past for both GNU Linux and MacOS OSX

Pass the number of days in the past for which you want the date. If you pass nothing it will assume 0 days.

date_days_past () { days_past=${1:-0}; if ! date -v-${days_past}d +%Y%m%d 2>/dev/null; then date --date="-${days_past} day" +%Y%m%d; fi }

Set timezone and clock on Linux

1. su to root

2. Find your timezone file under /usr/share/zoneinfo. For example:

   /usr/share/zoneinfo/US/Pacific

3. OPTIONAL: backup the current timezone configuration:

   cp /etc/localtime /etc/localtime.old

4. Create a symbolic link from the appropiate timezone to /etc/localtime.

   ln -sf /usr/share/zoneinfo/US/Pacific /etc/localtime

or

   ln -sf /usr/share/zoneinfo/America/Los_Angeles /etc/localtime

5. Use rdate or ntpdate to set the system time (ntpdate is better):

   /usr/sbin/ntpdate time.nist.gov

or

   /usr/bin/rdate time.nist.gov

6. Set the hardware clock to the system time:

   /sbin/hwclock --systohc

Correct for clock drift

For where its not possible to run ntpd you may use adjtimex to correct for systematic drift.

aptitude install adjtimex

NIST

time.nist.gov

Pacific time
http://www.time.gov/timezone.cgi?Pacific/d/-8

ntp.org

pool.ntp.org

Query:

ntpdate -q -b pool.ntp.org

Query and set local System Clock:

ntpdate -b pool.ntp.org

hwclock

Something was corrupting the registers in an I2C RTC clock chip I was using. I was getting this error after the chip registers were corrupt:

# hwclock
The Hardware Clock registers contain values that are either invalid (e.g. 50th day of month) or beyond the range we can handle (e.g. Year 2095).

I ran `strace` with -e raw=ioctl option because I needed the ioctl constant for RTC_RD_TIME, 0x40247009.

# strace hwclock
...
open("/dev/rtc", O_RDONLY|O_LARGEFILE)  = -1 ENOENT (No such file or directory)
open("/dev/rtc0", O_RDONLY|O_LARGEFILE) = 3
stat64(0x1000850c, 0xbfb87c60)          = -1 ENOENT (No such file or directory)
ioctl(3, RTC_UIE_ON, 0)                 = -1 ENOTTY (Inappropriate ioctl for device)
ioctl(3, RTC_RD_TIME, {tm_sec=7, tm_min=41, tm_hour=27, tm_mday=29, tm_mon=10, tm_year=265, ...}) = 0
ioctl(3, RTC_RD_TIME, {tm_sec=7, tm_min=41, tm_hour=27, tm_mday=29, tm_mon=10, tm_year=265, ...}) = 0
...
ioctl(3, RTC_RD_TIME, {tm_sec=7, tm_min=41, tm_hour=27, tm_mday=29, tm_mon=10, tm_year=265, ...}) = 0
ioctl(3, RTC_RD_TIME, {tm_sec=8, tm_min=41, tm_hour=27, tm_mday=29, tm_mon=10, tm_year=265, ...}) = 0
ioctl(3, RTC_RD_TIME, {tm_sec=8, tm_min=41, tm_hour=27, tm_mday=29, tm_mon=10, tm_year=265, ...}) = 0
open("/etc/localtime", O_RDONLY)        = 4
fstat64(0x4, 0xbfb87980)                = 0
...
# strace -e raw=ioctl hwclock
...
open("/dev/rtc", O_RDONLY|O_LARGEFILE)  = -1 ENOENT (No such file or directory)
open("/dev/rtc0", O_RDONLY|O_LARGEFILE) = 3
stat64(0x1000850c, 0xbfe5cc60)          = -1 ENOENT (No such file or directory)
ioctl(0x3, 0x20007003, 0)               = -1 (errno 25)
ioctl(0x3, 0x40247009, 0xbfe5c988)      = 0
ioctl(0x3, 0x40247009, 0xbfe5c9b4)      = 0
...
ioctl(0x3, 0x40247009, 0xbfe5c9b4)      = 0
ioctl(0x3, 0x40247009, 0xbfe5cb10)      = 0
ioctl(0x3, 0x40247009, 0xbfe5cb10)      = 0
open("/etc/localtime", O_RDONLY)        = 4
fstat64(0x4, 0xbfe5c980)                = 0
...
# cat /etc/adjtime
-1.865802 1277906845 0.000000
1277906845
UTC

Time Sync radio broadcasts

Most consumer "atomic" clocks use WWVB for radio sync. Time signals can also be received through GPS receivers which pickup the time signals broadcast by GPS satellites. Each GPS satellite carries its own atomic clock.

WWVB radio broadcasts of UTC time.

WWVB, Fort Collins, CO
60 kHz (binary carrier signal)
WWV, Fort Collins, CO
2500, 5000, 10000 and 15000 kHz (broadcasts voice)
WWVH, Kauai, HI
2500, 5000, 10000 and 15000 kHz (broadcasts voice)
CHU, Ottawa, Ontario, Canada
3330, 7335 and 14670 kHz (broadcasts voice)

UTC

UTC(GMT)EDTEST / CDTCST / MDTMST / PDTPST
00008 PM7 PM6 PM5 PM4 PM
01009 PM8 PM7 PM6 PM5 PM
020010 PM9 PM8 PM7 PM6 PM
030011 PM10 PM9 PM8 PM7 PM
0400MIDNIGHT11 PM10 PM9 PM8 PM
05001 AMMIDNIGHT11 PM10 PM9 PM
06002 AM1 AMMIDNIGHT11 PM10 PM
07003 AM2 AM1 AMMIDNIGHT11 PM
08004 AM3 AM2 AM1 AMMIDNIGHT
09005 AM4 AM3 AM2 AM1 AM
10006 AM5 AM4 AM3 AM2 AM
11007 AM6 AM5 AM4 AM3 AM
12008 AM7 AM6 AM5 AM4 AM
13009 AM8 AM7 AM6 AM5 AM
140010 AM9 AM8 AM7 AM6 AM
150011 AM10 AM9 AM8 AM7 AM
1600NOON11 AM10 AM9 AM8 AM
17001 PMNOON11 AM10 AM9 AM
18002 PM1 PMNOON11 AM10 AM
19003 PM2 PM1 PMNOON11 AM
20004 PM3 PM2 PM1 PMNOON
21005 PM4 PM3 PM2 PM1 PM
22006 PM5 PM4 PM3 PM2 PM
23007 PM6 PM5 PM4 PM3 PM