[DUG] Timezones for Bills and things

Fri Nov 25 14:24:11 NZDT 2016

Different filesystem types also have different time granularity – eg FAT32 and NTFS, one way this manifests is as if a utility such as XCOPY /U  to copy later files from one volume type to another, and if the same command is repeated the same files may be copied again, because the timestamps are slightly different.

FAT - 2 seconds
exFAT - 10 milliseconds
NTFS - 100 nanoseconds

Changes in DST are processed differently also depending on whether the PC has been rebooted since the last DST time change

To give a hint at the complexity - more info from

https://msdn.microsoft.com/en-us/library/windows/desktop/ms724290(v=vs.85).aspx
A file time is a 64-bit value that represents the number of 100-nanosecond intervals that have elapsed since 12:00 A.M. January 1, 1601 Coordinated Universal Time (UTC). The system records file times when applications create, access, and write to files.

The NTFS file system stores time values in UTC format, so they are not affected by changes in time zone or daylight saving time. The FAT file system stores time values based on the local time of the computer. For example, a file that is saved at 3:00pm PST in Washington is seen as 6:00pm EST in New York on an NTFS volume, but it is seen as 3:00pm EST in New York on a FAT volume.

Time stamps are updated at various times and for various reasons. The only guarantee about a file time stamp is that the file time is correctly reflected when the handle that makes the change is closed. 

Not all file systems can record creation and last access times, and not all file systems record them in the same manner. For example, the resolution of create time on FAT is 10 milliseconds, while write time has a resolution of 2 seconds and access time has a resolution of 1 day, so it is really the access date. The NTFS file system delays updates to the last access time for a file by up to 1 hour after the last access.

From: Ross Levis 
Sent: Friday, November 25, 2016 12:57 PM
To: 'NZ Borland Developers Group - Delphi List' 
Subject: Re: [DUG] Timezones for Bills and things

Yep, so some devices are showing daylight savings time and others are not for the same physical file, proving the file is stored in UTC and being displayed in local time.

From: delphi-bounces at listserver.123.net.nz [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Rohit Gupta
Sent: Friday, 25 November 2016 10:48 a.m.
To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] Timezones for Bills and things

Not quite.  Because I am not referring to the actual timestamp that it is showing me.  But the fact that between the 5pcs, a laptop and surface and usb harddisk.  The timestamps are different twice a year.  If it was storing them as utc then they would all be always in sync with each other.  They would all change by the same hour and look the same in beyond compare.  But different files change their timestamps on different pcs.  If its storing them as utc then it is selective about it.  Which is worse than not saving them as utc.  

Rohit

On 25/11/2016 10:06, David Brennan wrote:

  Hi,

  Surely that is evidence that Windows is storing timestamps in UTC? If it wasn’t then a file stamped as 3pm would always say 3pm no matter what timezone you changed your computer to.

  Regards,

  David.

  From: delphi-bounces at listserver.123.net.nz [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Rohit Gupta
  Sent: Friday, 25 November 2016 9:59 a.m.
  To: NZ Borland Developers Group - Delphi List mailto:delphi at listserver.123.net.nz
  Subject: Re: [DUG] Timezones for Bills and things

  Thanks Ross,

  however I would dispute that windows stores file timestamps in utc.  Because twice a year there is mayhem as only some files change their timestamp by an hour.  And these files are different on different workstations.

  Maybe it's true for the server version of windows.

  Rohit

  On 24/11/2016 17:39, Ross Levis wrote:

    I suggest store in UTC, like Windows does for file timestamps, and use Windows functions like GetTimeZoneInformation and SystemTimeToTzSpecificLocalTime to convert to locale time for display/printing purposes.  A timezone database shouldn’t be required.

    From: delphi-bounces at listserver.123.net.nz [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Stefan Mueller
    Sent: Thursday, 24 November 2016 5:21 p.m.
    To: 'NZ Borland Developers Group - Delphi List'
    Subject: Re: [DUG] Timezones for Bills and things

    https://github.com/pavkam/tzdb has such a timezone database (2014 data). Using that library makes it easy to convert between timezones.

    I think http://www.iana.org/time-zones hosts the latest up to date time zone data that this tzdb database is based on in case you want to update it to the latest 2016 data. 

    Or you could rewrite your application to use Oracle Database … they do have a timestamp datatype for your table columns so when you select/insert/update a table all you need to do is to set the correct locale and the database will do the conversion for you.

    https://docs.oracle.com/database/121/NLSPG/ch4datetime.htm#NLSPG263

    Kind regards,

    Stefan Müller,
    R&D Manager

    ORCL Toolbox Ltd. 
    Auckland, New Zealand 

    P Please consider the environment before printing this email

    This message is intended for the adresse named above and may contain privileged or confidential information.
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    From: delphi-bounces at listserver.123.net.nz [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Rohit Gupta
    Sent: Thursday, 24 November 2016 4:19 p.m.
    To: NZ Borland Developers Group - Delphi List
    Subject: [DUG] Timezones for Bills and things

    We finally have to deal with time zones.  We use the server date-time everywhere, rather than relying on workstation date-times.  This is not an interactive forum type application where the flow of data is merged from various timezones.  It is a business management system with appointments and bills etc.  The timestamps of these records have to remain as created.  But they can not come from the workstation clock.

    Going forward, a database server could be servicing workstations in different time zones.

    I am considering

      1.. Set the server time as UTC 
      2.. Keep a table for time zone versus branch (how do I keep this upto date at daylight saving boundaries) 
      3.. Use the utc + timezone difference to stamp each bill or appointment made for each branch.
    How is everyone else handling it ?

    Regards

    Rohit

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  -- 

  Regards

  Rohit Gupta
  B.E. Elec., M.E., Mem IEEE, Mem IET
  Technical Director
  Computer Fanatics Ltd

  Tel 4892280 
  Fax 4892290 
  Web www.cfl.co.nz

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Regards

Rohit Gupta
B.E. Elec., M.E., Mem IEEE, Mem IET
Technical Director
Computer Fanatics Ltd

Tel 4892280 
Fax 4892290 
Web www.cfl.co.nz

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