
Hi, 
Are you sure these are date objects and not strings? For example

> d1<-runif(10,0,100)
> d2<-runif(10,0,200)
> df<-data.frame(d1=d1+as.Date("2010-6-20")+d1,d2=as.Date("2009-6-20")+d2)
> df
           d1         d2
1  2010-09-23 2009-06-30
2  2010-06-25 2009-08-21
3  2010-10-10 2009-08-04
4  2010-07-06 2009-07-30
5  2010-07-05 2009-10-19
6  2010-07-07 2009-08-01
7  2010-11-09 2009-09-25
8  2010-07-19 2009-12-03
9  2010-12-24 2009-07-19
10 2010-09-25 2009-06-27
> df$DD<-df$d1-df$d2
> df
           d1         d2            DD
1  2010-09-23 2009-06-30 449.7866 days
2  2010-06-25 2009-08-21 307.0807 days
3  2010-10-10 2009-08-04 431.7428 days
4  2010-07-06 2009-07-30 341.2171 days
5  2010-07-05 2009-10-19 259.0490 days
6  2010-07-07 2009-08-01 339.7433 days
7  2010-11-09 2009-09-25 410.1609 days
8  2010-07-19 2009-12-03 227.1901 days
9  2010-12-24 2009-07-19 523.5030 days
10 2010-09-25 2009-06-27 454.6529 days

generates differences between the 2 columns without NAs. 
What's the output you get when you call str on your data frame?

Christos 


> Date: Sun, 20 Jun 2010 19:13:24 -0500
> From: erinm.hodgess@gmail.com
> To: r-help@stat.math.ethz.ch
> Subject: [R]  difference in dates
> 
> Dear R People:
> 
> I have a data frame with the two following date columns:
> 
> 
> > a.df[1:10,c(1,6)]
>            DATE      DEATH
> 1207 2009-04-16 2009-05-06
> 1514 2009-04-16 2009-05-06
> 2548 2009-04-16 2009-05-08
> 3430 2009-04-16 2009-05-09
> 3851 2009-04-16 2009-05-09
> 3945 2009-04-16 2009-05-09
> 7274 2009-04-16 2009-05-12
> 7532 2009-04-16 2009-05-12
> 7651 2009-04-16 2009-05-12
> 8495 2009-04-16 2009-05-13
> >
> 
> I would like to generate a 3rd column which is the difference in days
> between the two dates.
> 
> I tried
> 
> a.df$DEATH - a.df$DATE
> 
> but I obtain all NA.
> 
> Any help would be much appreciated.
> 
> Thanks,
> Erin
> 
> 
> 
> -- 
> Erin Hodgess
> Associate Professor
> Department of Computer and Mathematical Sciences
> University of Houston - Downtown
> mailto: erinm.hodgess@gmail.com
> 
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