R help - Mar 2011 - Difference in numeric Dates between Excel and R

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?

Thank you
Felipe Parra

	[[alternative HTML version deleted]]

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Luis Felipe Parra
> Sent: Tuesday, March 01, 2011 3:07 PM
> To: r-help
> Subject: [R] Difference in numeric Dates between Excel and R
> 
> 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?
> 
I think so.  It is a known problem that Excel thinks 1900 was a leap year, but
it was not.  So Excel counts an extra day (for nonexistent Feb 29, 1900).  In
addition,  Excel considers "1900-01-01" as day 1, not day 0.

Hope this is helpful,

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204

On 2/03/2011 12:31 p.m., Nordlund, Dan (DSHS/RDA) wrote:
>> -----Original Message----- From: r-help-bounces at r-project.org
>> [mailto:r-help-bounces at r- project.org] On Behalf Of Luis Felipe
>> Parra Sent: Tuesday, March 01, 2011 3:07 PM To: r-help Subject: [R]
>> Difference in numeric Dates between Excel and R
>>
>> 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?
>>
>
> I think so.  It is a known problem that Excel thinks 1900 was a leap
> year, but it was not.  So Excel counts an extra day (for nonexistent
> Feb 29, 1900).  In addition,  Excel considers "1900-01-01" as day
1,
> not day 0.
>
> Hope this is helpful,
>
> Dan
An explanation which seems reasonably authoritative is given here:
http://www.cpearson.com/excel/datetime.htm


David Scott
>
> Daniel J. Nordlund Washington State Department of Social and Health
> Services Planning, Performance, and Accountability Research and Data
> Analysis Division Olympia, WA 98504-5204
>
>
> ______________________________________________ R-help at r-project.org
> mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do
> read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

-- 
_________________________________________________________________
David Scott	Department of Statistics
		The University of Auckland, PB 92019
		Auckland 1142,    NEW ZEALAND
Phone: +64 9 923 5055, or +64 9 373 7599 ext 85055
Email:	d.scott at auckland.ac.nz,  Fax: +64 9 373 7018

Director of Consulting, Department of Statistics

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]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
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