[R] help working with date values

[Uwe Ligges]

Steve_Friedman at nps.gov wrote:
> 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" 


year <- as.numeric(format((strptime(observation_date, "%Y-%m-%d")), "%Y"))

or

year <- as.numeric(substr(observation_date, 1, 4))


Uwe Ligges


> 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
> 
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