One of the fields in struct stat is st_mtime. I assume that is seconds since jan 1, 1970. Is that GMT or local time?
The time_t type represents the number of seconds that have passed since 1 January 1970 00:00 UTC (that moment in time is called the "epoch" and happened at the same moment everywhere around the world). You can consider "UTC" to mean the same thing as "GMT" (see Leap Second for detail about the very small differences).
Be aware that instead of adding or subtracting values from the time_t type, you should always use the localtime() and mktime() functions to convert to and from a local time zone representation.
Related
When running the following command with GNU date :
date -d "20145-01-23"
I get the following result :
date: invalid date `20145-01-23'
However, the manual states that year can be any number :
For numeric months, the ISO 8601 format ‘year-month-day’ is allowed, where year is any positive number [...]
Thus, two questions :
is this a bug ? or am I misunderstanding something ?
how to obtain the same behavior than BSD date, which accepts this as a valid date ?
This works for me, using GNU coreutils 8.24:
$ date -d "20145-01-23"
Sat Jan 23 00:00:00 PST 20145
I'm not certain of this explanation (UPDATE: looks like my guess was correct), but my guess is that you're on a system with a 32-bit time_t rather than 64 bits. With 32 bits, the only representable dates are from Fri 1901-12-13 20:45:52 UTC (231 seconds before the epoch) to Tue 2038-01-19 03:14:07 UTC (231-1 seconds after the epoch).
If this is the case, then this command should succeed:
$ date -d 2037-01-01
Thu Jan 1 00:00:00 PST 2037
and this one should fail:
$ date -d 2039-01-01
date: invalid date ‘2039-01-01’
(I've reconstructed what the result should look like for that last command. The output of the first will vary slightly depending on your time zone.)
how to obtain the same behavior than BSD date, which accepts this as a valid date ?
Are you saying you want BSD date to reject it? If so (more speculation), either run a 32-bit version of BSD, or test whether the requested time is outside the 32-bit range of timestamps.
BSD date -d expects a time zone value. To offset years, days, or whatever, you pass -vNT, where N is a number and T is a char to indicate days, months, or years.
The flags differ. See http://www.unix.com/man-page/All/1/date/ for the Gnu version, http://www.unix.com/man-page/FreeBSD/1/date/ for a common BSD variant.
I want to know that is there any command which can provide time without DST if DST is applicable in the zone.
I have searched lot in google but not getting proper answer. I think there should be simple solution to get it.
Below is one link on stackoverflow.com but I am not getting
https://unix.stackexchange.com/questions/123493/disable-daylight-saving-time-in-debian-linux
For example:
current time in Newyork is
date
Wed Mar 23 04:51:54 EDT 2016
As per DST-free timezone definitions provided which just define the GMT-offset, called Etc/GMT±X:
TZ=Etc/GMT-1 date
Wed Mar 23 10:13:09 GMT-1 2016
Whereas DST is 1 hour forward on March 23 i.e. it should be Wed, Mar 23, 2016, 4:13 AM
Please anyone provide help.
U.S. Eastern time is often represented by the time zone name "America/New_York" or "US/Eastern". Equivalently, there is the "Posix" time zone name "EST5EDT". The essential fact here is that this zone is nominally 5 hours off of UTC (or 4 hours when daylight saving time is in effect).
There are also some DST-free zone names of the form "UTC-4" and "UTC+5".
So if you say
export TZ="UTC+5"
date
You'll see the date in the equivalent of U.S. Eastern Standard Time, without a DST correction.
(This is essentially what the high-rated answer at https://unix.stackexchange.com/questions/123493/disable-daylight-saving-time-in-debian-linux was trying to tell you, I think.)
If you wanted to take an arbitrary time zone name and construct from it an equivalent DST-free zone name, that'd be pretty tricky.
I have the following small php snippet running on a gentoo Linux (php version 5.2.10-pl0-gentoo):
#!/usr/bin/php5
<?
class TestDaemon {
public function __construct(){
while (TRUE){
unset($aDate);
exec("date", $aDate);
print("date(\"d.m.y H:i:s\") yields: ".date("d.m.y H:i:s")." while 'date' yields $aDate[0].\n");
sleep(1);
}
}
}
$oDaemon = new TestDaemon();
?>
And the output produced is as follows:
date("d.m.y H:i:s") yields: 27.03.14 07:05:27 while 'date' yields Thu Mar 27 07:05:03 UTC 2014.
date("d.m.y H:i:s") yields: 27.03.14 07:05:28 while 'date' yields Thu Mar 27 07:05:04 UTC 2014.
date("d.m.y H:i:s") yields: 27.03.14 07:05:29 while 'date' yields Thu Mar 27 07:05:05 UTC 2014.
date("d.m.y H:i:s") yields: 27.03.14 07:05:30 while 'date' yields Thu Mar 27 07:05:06 UTC 2014.
date("d.m.y H:i:s") yields: 27.03.14 07:05:32 while 'date' yields Thu Mar 27 07:05:07 UTC 2014.
date("d.m.y H:i:s") yields: 27.03.14 07:05:33 while 'date' yields Thu Mar 27 07:05:09 UTC 2014.
date("d.m.y H:i:s") yields: 27.03.14 07:05:34 while 'date' yields Thu Mar 27 07:05:10 UTC 2014.
date("d.m.y H:i:s") yields: 27.03.14 07:05:35 while 'date' yields Thu Mar 27 07:05:11 UTC 2014.
As you can see the times are off by approx. 24 seconds. On a different machine (same OS, same version of PHP) I do not see such an offset.
What is the reason for this offset? Does this come from leap second differences? Then which system gives the correct time? Why does php not use the Linux system time instead?
Also, can this time offset be a source of problems when working with the mysql database on the same system?
This looks like the shell running /bin/date is configured to use the "right" timezones and php is configured to use the POSIX-conformant timezones. The difference now should be 25 seconds, but if the tz data is over two years old then it would be 24 seconds. For a picture of why visit http://www.ucolick.org/~sla/leapsecs/amsci.html and see the second plot. The "right" zones follow the green line. The POSIX zones are required to stop the system clock on every leap second, so they follow the descending staircase of the blue line. [edit to be sure which was using which method]
Why the offset I don't know, but it seems that php date() uses time() function to get a timestamp. time() is defined like this:
Returns the current time measured in the number of seconds since the Unix Epoch (January 1 1970 00:00:00 GMT).
it never says in documentation that it uses System time.
According to this SO post the function time() uses the date.timezone set in php.ini or date_default_timezone_set().
So if your system uses a different time zone than your php.ini it could explain the difference because php seems to use its own time.
He also propose the following solution to get the real system time if they are not the same:
I'm going to give you a solution that works for Linux, I don't know for Windows. In Linux the system timezone is set in /etc/timezone.
Now, this is normally outside my allowed open_basedir setting, but you can add :/etc/timezone to your list to be able to read the file.
Then, on top of the scripts, that want to get the system time, you can call a library function that sets the script timezone to the system timezone. I suppose that this function is part of a class, so I use static:
static function setSystemTz() {
$systemTz = trim(file_get_contents("/etc/timezone"));
if ($systemTz == 'Etc/UTC') $systemTz = 'UTC';
date_default_timezone_set($systemTz);
}
To make the matter worse in PHP 5.3.3 'Etc/UTC' is not recognized, while 'UTC' is, so I had to add an if to fix that.
Not really answering your question but there are many different types of time variable on a typical Linux system:
The internal time of the harware clock (aka RTC or BIOS time), if you have one. This keeps the time when the system is offline, this may be fake and return bad values on a virtual machine, or you might not even have one. See hwclock
The amount of time that has passed since the kernel was started. (cat /proc/uptime)
The external time as provided by an NTP server via ntpd.
The innacuracy of the realtime clock / time source, how much it drifts over time in parts per million between samples. This is set by calls to the kernel. On a system with NTP it will be saved to your drift file (/var/lib/ntp/drift/ntp.drift or similar) and the system's time will be smoothly adjusted rather than have time jumps in your logs.
The number of miliseconds since 1st of January 1970. See date +%s
Your timezone, set in /etc/localtime, see zdump /etc/localtime
The current system time, calculated using all of the above plus rules for timezone, leap year, leap second and so on. See date
If I had to guess, I'd say that NTP is slowly adjusting your system time to compensate for your skewed realtime clock while PHP bypasses this and snags it from somewhere else.
I have a situation where the relative time is more important to a user than an absolute time. So it's more important to be able to quickly say "event happened 5 days and 5 hours ago" than "event happened at 1 PM CDT and it's 5 PM CST 5 days later now."
We store our dates in UTC and convert to display for the user:
pDateTime = DateTime.SpecifyKind(pDateTime, DateTimeKind.Utc);
DateTimeZone dateTimeZone = DateTimeZoneProviders.Tzdb[pCurrentUser.PreferredTimezone];
return Instant.FromDateTimeUtc(pDateTime).InZone(dateTimeZone).ToString("HH':'mm':'ss' 'MM'/'dd'/'yy' 'x", CultureInfo.InvariantCulture);
We'll be using NodaTime 1.2 when it's fully out and just used vanilla ToString before.
However, times using this pattern end up using the daylight status of the time as opposed to the current daylight status. This means that times look something like: 16:15:32 10/25/13 CDT even though we have now transitioned to CST.
It is an absolute measure of the time. This forces the user to do the logic: "How long ago was that? Is it daylight saving time now? If so, the difference is x. If not, I have to add or subtract an hour? That makes the difference y."
Meanwhile, a relative measure of the time would display 15:15:32 10/25/13 CST in the absence of DST. This forces the user to do no conversions and allows them to compute what that time means in context much easier.
In a display that has numerous dates, it can get tricky to do the absolute time logic over the entire set. Doing it once is tricky to get right. However, a friendly relative string like "posted 5 hours ago" also forces them to resolve both the date and time themselves - that information is still important.
A compromise might be to do the posted blank hours/minutes ago for the first 24 hours or to include both the friendly string and absolute time - these are both patterns I've seen done.
But ignoring those, is there a way in NodaTime to imbue a time with a specific daylight status in order to get times displaying in a relative context?
However, times using this pattern end up using the daylight status of the time as opposed to the current daylight status. This means that times look something like: 16:15:32 10/25/13 CDT even though we have now transitioned to CST.
Yes, and it should. Displaying a date/time with CST despite that date/time occurring in CDT would be very odd, IMO.
So it's more important to be able to quickly say "event happened 5 days and 5 hours ago" than "event happened at 1 PM CDT and it's 5 PM CST 5 days later now."
In that case you shouldn't be displaying a date/time at all, in my view. Convert both ZonedDateTime values to Instant, take the Duration between them, and then you can see that it's 5 days and 5 hours ago. (I can't remember how much help we provide with that - you may need to manually take the number of ticks and divide by NodaConstants.TicksPerStandardDay etc. Look at DurationPattern to see if it helps though.)
Alternatively, if you really want to display a date and time, but still easily be able to extract the difference between them mentally, two options suggest themselves:
Use OffsetDateTime instead; there you could force the offsets to be the same, although I still think it would be odd to display an offset which wasn't actually the current offset in the zone you were observing the time in. Or you could just display the relevant offset at the time, so -5 for CST and -4 for CDT.
Just display everything in UTC, so that daylight saving transitions are irrelevant.
Note that you can't get months between the two ZonedDateTime values, as we're dealing with an elapsed time (a duration) rather than calendar-logical arithmetic (a period).
pytz's documentation says:
Note that this instance [pytz.timezone('UTC')] is not the same instance (or implementation)
as other timezones with the same meaning (GMT, Greenwich, Universal,
etc.).
and indeed:
>>> pytz.timezone('UTC') is pytz.timezone('GMT')
False
So... what's the difference?
When should I use pytz.timezone('UTC') and when should I use pytz.timezone('GMT')?
The UTC implementation is a tzinfo implementation that will at all times return 0 minutes offset.
The timezone you get when you do pytz.timezone('GMT') is the GMT timezone defined in the Olson database. It also will return 0 at all times, but in a more complicated manner.
Use UTC.