Question

How do I get the offset from GMT of the local time (accounting DST)

Asked by: sciuriware

I would like to correct the modification time of files to GMT
in order to overcome daylight saving time changes.
As you may know Windows 'shows' a different time then.

I have:

            STAT stat_buffer;
                time_t mod_time;

                  _stat(current_file, &stat_buffer);
                       mod_time = stat_buffer.st_mtime;

How do I correct 'mod_time' into GMT to make it DST-independent?
Can I obtain a one-time offset to subtract?

;JOOP!

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Asked On
2009-10-18 at 04:14:51ID24821497
Tags

timezone C++

Topic

Java Programming Language

Participating Experts
1
Points
500
Comments
13

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Answers

 

by: CEHJPosted on 2009-10-18 at 18:15:05ID: 25602163

The following may be of use:

http://technojeeves.com/joomla/index.php/free/83-java-jna-touch

Get the Unix time by using a Calendar with TimeZone set to "UTC"

 

by: sciuriwarePosted on 2009-10-18 at 22:34:20ID: 25602769

May be a misunderstanding.
I want to record the timestamp of various files in order to see if they were modified
over a long period.
If I can correct that timestamp to UTC I can do this crossing DST switching.
I don't have to change the modification time of the file.

Or, I want to know: is it DST at present or not,
so I can subtract an offset of all the times sampled.

;JOOP!

 

by: CEHJPosted on 2009-10-19 at 01:24:31ID: 25603375

Well you can see the timestamp of the file in UTC by doing something like:

        File f = new File("newf");
        long lastMod = f.lastModified();
        DateFormat utc = DateFormat.getInstance();
        utc.setTimeZone(TimeZone.getTimeZone("UTC"));
        System.out.println(utc.format(new Date(lastMod)));
                                              
1:
2:
3:
4:
5:

Select allOpen in new window

 

by: sciuriwarePosted on 2009-10-19 at 01:33:18ID: 25603416

CEHJ, why do you think I asked this question in the C++ corner?
JAVA would not raise these problems for me,
but in the 90's I programmed in Visual C++.
I must update a few of those ancient applications before I completelty forget about C.

;JOOP!

 

by: CEHJPosted on 2009-10-19 at 01:46:36ID: 25603482

Sorry - didn't know you were looking for native code

 

by: CEHJPosted on 2009-10-19 at 02:01:33ID: 25603542

If you call localtime, you will get *tm back and tm-> tm_isdst will tell you if you're in DST

 

by: CEHJPosted on 2009-10-19 at 02:02:20ID: 25603545

(Input to localtime would be a time_t from a call to stat on the file)

 

by: sciuriwarePosted on 2009-10-19 at 02:03:10ID: 25603550

It's OK; be happy to know JAVA.
All my C++ app's were generated in VC; you get sick when you read the code.
Reasons enough to do away with those personal legacies.
But for now I have only one week to repair for DST.

;JOOP!

 

by: CEHJPosted on 2009-10-19 at 02:18:04ID: 25603623

Well that's how i'd do it anyway. Your offset from UTC would be in tm_gmtoff in tm

 

by: sciuriwarePosted on 2009-10-19 at 05:16:34ID: 25604442

'tm_gmtoff' :: unknown member.
But 'tm_isdst' exists.

;JOOP!

 

by: CEHJPosted on 2009-10-19 at 05:23:02ID: 25604476

Depending on the platform, that could be __tmgmtoff afaik.

 

by: sciuriwarePosted on 2009-10-19 at 05:36:07ID: 25604557

I just subtracted tm_hour.
But careful: struct tm is re-used so, save the values.

What a language!

;JOOP!

 

by: sciuriwarePosted on 2009-10-19 at 05:36:48ID: 31642590

JAVA or bust!

20120131-EE-VQP-002

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