[egenix-users] DateTime error rounding seconds

[M.-A. Lemburg]

Edwin Grubbs wrote:
> 
> On Tue, 2 Apr 2002, M.-A. Lemburg wrote:
> 
> > Edwin Grubbs wrote:
> > >
> > > I am having a problem with DateTime incorrectly rounding the floating
> > > point second value down 1/100 of second.
> > That's because 5.05 is really 5.0499999999999998. mxDateTime's
> > repr() function truncates the seconds value to avoid accidental
> > cases of a 60 reading when you really only have 59.6 seconds.
> >
> > Use now().second to access the true seconds value with
> > full accuracy.
> 
> The real problem with it, is that str() returns the truncated date just
> like repr(), and we are using str() to insert datetime fields. If you
> create a DateTime object for 03:04:05.02 and insert it into a Postgres 7
> database using its str() value, it is stored as 03:04:05.01. Then when you
> retrieve 03:04:05.01 from the database, the DateTime object shows it as
> 03:04:05.00. Then you cannot match the row that you just retrieved with
> its datetime column. How is the DateTime object supposed to be inserted
> and retrieved from the database?

The database interface should use the C API for this. It preserves
the accuracy all the way. If that's not an option, I'd consider using
the tuple representation now().tuple() which mimics 
time.localtime(time.time()) (but with added seconds accuracy).
 
> I understand that a float cannot represent most decimal values exactly,
> but truncating just increases the inaccuracy even though it might seem
> more coherent to have "2002-03-28 10:59:59.9999" truncated to "2002-03-28
> 10:59:59.99" instead of rounded to "2002-03-28 11:00:00.00".

Hmm, one possibility would be to use rounding for all second
values < 59.995 and apply truncation to all others.

-- 
Marc-Andre Lemburg
CEO eGenix.com Software GmbH
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