[egenix-users] DateTimeFromString off by one sec?

[M.-A. Lemburg]

On 2008-05-07 10:33, M.-A. Lemburg wrote:
> On 2008-05-06 23:27, Francesco Pierfederici wrote:
>> Hi,
>>
>> I am using mx.DateTime to parse dates and times in ISO format. I have
>> noticed that DateTimeFromString seems to be off by one second if the
>> time string being parsed ends with :59
>>
>>>>> mx.DateTime.Parser.DateTimeFromString('2008-05-06T19:30:09.59')
>> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.58' at 651a8>
>>
>> but
>>
>>>>> mx.DateTime.Parser.DateTimeFromString('2008-05-06T19:30:09.58')
>> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.58' at 8c250>
>>
>> Adding formats=['iso', ] does not change the results. I am using
>> mx.DateTime 3.0 by the way.
>>
>>
>> Am I doing something wrong?
> 
> It is not not off by a second, it's off by 1/100 of a second.
> 
> This looks a lot like a bug in the parser, since it starts to
> parse the fraction with a 1/100 second offset starting at
> x.59 onwards:
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.57')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.57' at 2b1e3bdb08c8>
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.58')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.58' at 2b1e3ba8bce8>
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.59')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.58' at 2b1e3bdb08c8>
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.60')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.59' at 2b1e3ba8bce8>
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.61')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.60' at 2b1e3bdb08c8>
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.62')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.61' at 2b1e3ba8bce8>
> 
> Note that it works again, if you use three decimal places:
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.591')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.59' at 2b1e3bdb08c8>
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.601')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.60' at 2b1e3bdb08c8>
> 
> However:
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.590')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.58' at 2b1e3ba8bce8>
> 
>  >>> DateTimeFrom('2008-05-06 19:30:09.600')
> <mx.DateTime.DateTime object for '2008-05-06 19:30:09.59' at 2b1e3bdb08c8>
> 
> Weird bug :-)

It appears to be a classical float rounding problem:

 >>> DateTimeFrom('2008-05-06 19:30:09.58').second
9.5800000000000001

 >>> DateTimeFrom('2008-05-06 19:30:09.59').second
9.5899999999999999

 >>> DateTimeFrom('2008-05-06 19:30:09.60').second
9.5999999999999996

 >>> DateTimeFrom('2008-05-06 19:30:09.61').second
9.6099999999999994

The value is parsed correctly using float():

 >>> float('09.58')
9.5800000000000001

 >>> float('09.59')
9.5899999999999999

 >>> float('09.60')
9.5999999999999996

 >>> float('09.61')
9.6099999999999994

It's just that the string formatting code truncates the
seconds value after two decimal places (to avoid
cases where it would print '19:30:60' using rounding).

I'm not sure how we could solve this. Perhaps we should
round for second values up to 59.99 (to two decimal places)
and truncate only for > 59.99 ?!

-- 
Marc-Andre Lemburg
eGenix.com

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