help working with date values

xx <- c("1985-09-02", "1985-09-16", "1985-07-31", "1985-07-31", "1985-09-02",

# 1
library(zoo)
as.numeric(floor(as.yearmon(xx)))

#2
as.numeric(substr(xx, 1, 4))

#3
as.numeric(format(as.Date(xx), "%Y"))

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