[DUG] DateTime Floating Point Errors.

[John Bird]

I am guessing you are keeping the milliseconds for a reason, but otherwise
throw them away - I lose the milliseconds every time I convert a date/time
unless I am actually using them for a timing loop.

  Eg  formatDateTime('dd mmm yyyy hh:nn:ss',dtDateTime);

I would imagine in general its good to be cautious about a test like

   if FloatVariable = AConstantValue

As there is so many ways it could fail if its not quite the same (for
instance even a different compiler version etc might cause code to fail..)
(This is probably one of the reasons I still do lots of this sort of stuff
with integer and int64 variables.....but then I am a bit of a luddite from
way back)

There is a similar but unrelated effect with filedatetime, where if you use
a filedatetime on two files to check if a file needs to be copied if one has
been changed the check will often fail if the 2 files are stored on
different types of file system (eg NTFS and FAT32 for instance as these have
different resolutions for file time modifications), so a file copied from
one to the other doesn't end up with exactly the same modification
date/time.  To do it properly you need to allow two files with modification
times up to 10 seconds apart to be assumed to be the same (2 in theory but
10 is safer I found).  I have been meaning to write a routine to copy files
if the dates are different taking this into account...but haven't yet, I am
hoping someone else out there has and will pop up brightly and say here is
one!

John


-----Original Message-----
From: delphi-bounces at ns3.123.co.nz [mailto:delphi-bounces at ns3.123.co.nz] On
Behalf Of Kyley Harris
Sent: Sunday, 23 October 2005 7:26 p.m.
To: NZ Borland Developers Group - Delphi List
Subject: [DUG] DateTime Floating Point Errors.


I wonder if anyone else has noticed this irritating rounding issue on
datetimes.

Bear in mind that this is not a useful answer is you care about historical
dates. I only care about dates greater than the year 2000.

Problem. When comparing an "empty date" (meaning Date = 0.0) delphi behaves
differently on different computers meaning that my if varDateValue = 0.0 can
return false a lot of the time. This is also the same when comparing Dates
that have a zero time factor.

on my computer, EncodeDate(1899,12,30) + EncodeTime(0,0,0,0) does not return
0.0, but something along the lines of zero + an infintesimal fraction that
does not equal zero any more.

I solve my problem with the following method of forcing a date when setting
a date property.

  if DateTimeValue < ZeroDateThreshold /* EncodeDate(1900,1,1) */  then
  begin
    result := 0.0;
  end else
  begin
    result := RecodeMilliSecond(result,0);
  end;

This forces a removal of all these little tiny bits of rubbish on the end of
my datetimes that occurs when I convert my string dates "yyyymmddhhnnsszzz"
into real date etc.

anyone got better ideas? if not.. hope this helps anyone else.



--
Kyley Harris
Harris Software
+64-9-8455274
+64-21-671-821

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