[R] help working with date values

[Steve_Friedman at nps.gov]

My data contains a variable "observation_date"  and it contains values as:

1985-09-02
1985-09-15
1985-07-31
1985-09-02


I need to process data annually rather than daily, therefore I'm trying to
1) either extract the first 4 digits from this field and use them as a new
variable "year"  or 2) keep the variable as it is and process the analysis
using the first 4 digits of the observation_date field.

I'm not sure how to do either one of these approaches. I've looked in the
R-archive help pages, date, strsplit and a few others

> attach(gator)
> observation_date[1:10]
 [1] 1985-09-02 1985-09-16 1985-07-31 1985-07-31 1985-09-02 1985-08-26
1985-07-31 1985-08-26 1985-09-02 1985-09-16

> as.date(observation_date)
Error in as.date(observation_date)  : Cannot coerce to date format

> mode(observation_date)
  "numeric"

> y <- as.character(observation_date)
[1]  "1985-09-02" "1985-09-16" "1985-07-31" "1985-07-31" "1985-09-02"
"1985-08-26" "1985-07-31" "1985-08-26" "1985-09-02" "1985-09-16"

< y.date <- as.date(y)
 [1] <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA> <NA>  on and on
and on ...


> x <- strsplit(observation_date, "-")
Error in strsplit(observation_date, "-")  : non-character argument


All help is greatly appreciated.

Thanks
Steve



Steve Friedman Ph. D.
Spatial Statistical Analyst
Everglades and Dry Tortugas National Park
950 N Krome Ave (3rd Floor)
Homestead, Florida 33034

Steve_Friedman at nps.gov
Office (305) 224 - 4282
Fax     (305) 224 - 4147