Unix Date Difference Between 2 and 3 Months Explained

command-line-interfacedate

How is this possible and how do I deal with it? I'm making backup script that is dependent on Unix date and have discovered an interesting bug:

[root@web000c zfs_test]# date +%y-%m-%d --date='2 months ago'
14-04-01
[root@web000c zfs_test]# date +%y-%m-%d --date='3 months ago'
14-02-28
[root@web000c zfs_test]# date
Sun Jun  1 00:08:50 CEST 2014

Best Answer

You're seeing this behavior because of summer time (daylight saving time).

Because you are currently in summer time, where your clock is one hour ahead, when you ask for three months ago at just after midnight on the first of June, the time ends up being one hour "earlier" because it was not summer time three months ago.

The GNU date documentation suggests to work around this by using 12:00 noon and the 15th of the month as starting points, when asking for relative days or months, respectively. For example:

date +%y-%m-%d --date="$(date +%Y-%m-15) -3 month"

Related Solutions

How to Run a .bat File in a Scheduled Task Without a Window

You could run it silently using a Windows Script file instead. The Run Method allows you running a script in invisible mode. Create a .vbs file like this one

Dim WinScriptHost
Set WinScriptHost = CreateObject("WScript.Shell")
WinScriptHost.Run Chr(34) & "C:\Scheduled Jobs\mybat.bat" & Chr(34), 0
Set WinScriptHost = Nothing

and schedule it. The second argument in this example sets the window style. 0 means "hide the window."

Complete syntax of the Run method:

 object.Run(strCommand, [intWindowStyle], [bWaitOnReturn])

Arguments:

object: WshShell object.
strCommand: String value indicating the command line you want to run. You must include any parameters you want to pass to the executable file.
intWindowStyle: Optional. Integer value indicating the appearance of the program's window. Note that not all programs make use of this information.
bWaitOnReturn: Optional. Boolean value indicating whether the script should wait for the program to finish executing before continuing to the next statement in your script. If set to true, script execution halts until the program finishes, and Run returns any error code returned by the program. If set to false (the default), the Run method returns immediately after starting the program, automatically returning 0 (not to be interpreted as an error code).

How to tell if a file is older than 30 minutes on AIX

I can do it using just touch, test, and date. (tested on AIX 5.3.0.0)

First you need to create a file 30 minutes in the past (Unfortunately, this requires prior knowledge of the current timezone on the machine. But you may be able to work that into things if need be.)

In this example, the current timezone is EST5EDT (GMT-4). If you're lucky, the machine timezone will be set at GMT and you can just use TZ=00:30:

-bash-3.00# date
Mon 26 Mar 14:22:31 2012
-bash-3.00# touch -t `TZ=04:30 date '+%Y%m%d%H%M.%S'` foo
-bash-3.00# ls -al foo
-rw-r--r--   1 root     system            0 26 Mar 13:52 foo

Now use test to compare the dates:

-bash-3.00# test '/smit.log' -ot foo && echo 'smit.log is older than 30min'
smit.log is older than 30min

Best Answer

Related Solutions

How to Run a .bat File in a Scheduled Task Without a Window

How to tell if a file is older than 30 minutes on AIX

Related Topic