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Different values of datnumber for same date-time?

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Abhinav
Abhinav on 5 Aug 2018
Edited: dpb on 8 Aug 2018
When I convert given date-number '2011-01-06 0:45' by
datenum(2011-01-06 0:45,'yyyy-mm-dd HH:MM')
I get
734509.0313
as date-number.
But When I generate a vector of date-numbers using
begin_date_time=datenum('2011-01-06 00:00:00','yyyy-mm-dd HH:MM:SS');
str_next_date=strcat('2011-01-06',32,'00:',time_scale{ind_node},':00');
next_date_time=datenum(str_next_date,'yyyy-mm-dd HH:MM:SS');
interval=next_date_time-begin_date_time;
end_date_time=datenum('2011-12-31 00:00:00','yyyy-mm-dd HH:MM:SS');
full_data_time_steps=begin_date_time:interval:end_date_time;
The fourth date-number in the vector 'full_data_time_steps' is
734509.0312.
But it corresponds to the same date-time '2011-01-06 0:45'. I am not sure why this is happening and how to solve this? Any help is appreciated.

Accepted Answer

dpb
dpb on 6 Aug 2018
It's the result of floating point rounding in the two different ways you generated the datenum values --
In
full_data_time_steps=begin_date_time:interval:end_date_time;
the interval value is an approximation and over the vector it will accumulate noticeable differences.
The solution is to always use integer values of whatever granularity is needed to generate sequences of dates such that the internal calculations are reproducible.
Presuming you're using ML release R2014b or later use datetime instead of datenum; there are many features that make it more flexible.
  4 Comments
dpb
dpb on 6 Aug 2018
No problem; it's a very common issue and the doc doesn't do enough to explain the problem and how to avoid it.

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