Geoffrey:

There may be something for this in one of the packages dealing with dates. 

If not, here's one (incomplete) idea, based on something I used for a similar issue a little while ago. Essentially, make a data frame that ranks each weekday over a period in ascending order. This data frame could then be merged in with your data set, with the difference between the "date_num" values for any two dates equalling the number of weekdays between them.

start.date <- as.Date("2006-01-27")
end.date <- as.Date("2007-08-27")
date.list <- seq(from=start.date, to=end.date, by="day")
date.df <- data.frame(date=date.list, weekday=weekdays(date.list))
date.df <- subset(date.df, subset=!(weekday %in% c("Saturday", "Sunday")))
date.df$date_num <- 1:length(date.df$date)
date.df[1:20,]

> date.df[1:20,]
         date   weekday date_num
1  2006-01-27    Friday        1
4  2006-01-30    Monday        2
5  2006-01-31   Tuesday        3
6  2006-02-01 Wednesday        4
7  2006-02-02  Thursday        5
8  2006-02-03    Friday        6
11 2006-02-06    Monday        7
12 2006-02-07   Tuesday        8
13 2006-02-08 Wednesday        9
14 2006-02-09  Thursday       10
15 2006-02-10    Friday       11
18 2006-02-13    Monday       12
19 2006-02-14   Tuesday       13
20 2006-02-15 Wednesday       14
21 2006-02-16  Thursday       15
22 2006-02-17    Friday       16
25 2006-02-20    Monday       17
26 2006-02-21   Tuesday       18
27 2006-02-22 Wednesday       19
28 2006-02-23  Thursday       20

-Ian

On May 23, 2011, at 12:30 PM, Geoffrey Smith wrote:

> Thank you for the email.  The data is unbalanced, meaning that some days are
> missing.  So the sequence of days could be something like Tuesday,
> Wednesday, Friday, Monday.  The diff function would produce 1, 2, 3.  But I
> would like it to produce 1, 2, 1 since there is really only 1 day between
> Friday and Monday.  Thank you.  Geoff
> 
> On Mon, May 23, 2011 at 10:09 AM, Jeff Newmiller
> <jdnewmil@dcn.davis.ca.us>wrote:
> 
>> Your request seems bizarre to me. If you really want to ignore the actual
>> time intervals, just do your analysis without the actual dates.
>> 
>> If you just want forward-looking time intervals, then put them in the
>> correct index locations.
>> 
>> ?diff
>> 
>> c(diff(DF$DATE),1)
>> ---------------------------------------------------------------------------
>> Jeff Newmiller The ..... ..... Go Live...
>> DCN:<jdnewmil@dcn.davis.ca.us> Basics: ##.#. ##.#. Live Go...
>> Live: OO#.. Dead: OO#.. Playing
>> Research Engineer (Solar/Batteries O.O#. #.O#. with
>> /Software/Embedded Controllers) .OO#. .OO#. rocks...1k
>> ---------------------------------------------------------------------------
>> 
>> Sent from my phone. Please excuse my brevity.
>> 
> 
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> 
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