[R] Difference in numeric Dates between Excel and R

[Prof Brian Ripley]

On Wed, 2 Mar 2011, Luis Felipe Parra wrote:

> Hello. I am using some dates I read in excel in R. I know the excel origin
> is supposed to be 1900-1-1. But when I used as.Date with origin=1900-1-1 the
> dates that R reported me where two days ahead than the ones I read from
> Excel. I noticed that when I did in R the following:
>
>> as.Date("2011-3-4")-as.Date("1900-1-1")
> Time difference of 40604 days
>
> but if I do the same operation in Excel the answer is 40605. Does anybody
> know what can be going on?

We cannot know: you say a difference of 2 and report 1!

As the examples from as.Date says

      ## Excel is said to use 1900-01-01 as day 1 (Windows default) or
      ## 1904-01-01 as day 0 (Mac default), but this is complicated by Excel
      ## thinking 1900 was a leap year.
      ## So for recent dates from Windows Excel
      as.Date(35981, origin="1899-12-30") # 1998-07-05
      ## and Mac Excel
      as.Date(34519, origin="1904-01-01") # 1998-07-05

So the origin you used is off by 2 days: one for the origin being day 
1 and one for Windows Excel's ignorance of the calendar.

Note too that these are *default*: they can be changed in Excel.

> Thank you
> Felipe Parra
>
> 	[[alternative HTML version deleted]]
>
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PLEASE do try to do your own homework (and not send HTML), as we 
requested there.  It is galling that you ask here about bugs in Excel, 
bugs that are even documented in R's help.  In future, please use the 
Microsoft help you paid for with Excel if it disagrees with R.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595