[datatable-help] Integer date and time classes for use with data.table

[Short, Tom]

See enclosed for a draft of classes that implement dates and times with
integer storage. The IDate class is a simple wrapper around the Date
class that tries to keep an integer storage format. The ITime class, the
time of day, is stored as the number of seconds in a day.

Because IDate and ITime are stored as integers with ranges less than
100,000, data.table indexing and sorting is fast. Also included are
conversions to and from POSIXct and chron formats.

Comments and tests are welcome. 

Examples:

>     (t <- as.ITime("10:45:04"))
[1] "10:45:04"
>     (d <- as.IDate("2001-01-01"))
[1] "2001-01-01"
> 
>     datetime <- seq(as.POSIXct("2001-01-01"),
+                     as.POSIXct("2001-01-03"), by = "5 hour")
>     
>     (a <- data.table(IDateTime(datetime), a = rep(1:2, 5),
+                      key = "a,date,time"))
            date     time a
 [1,] 2001-01-01 00:00:00 1
 [2,] 2001-01-01 10:00:00 1
 [3,] 2001-01-02 06:00:00 1
 [4,] 2001-01-02 16:00:00 1
 [5,] 2001-01-02 20:00:00 1
 [6,] 2001-01-01 05:00:00 2
 [7,] 2001-01-01 15:00:00 2
 [8,] 2001-01-02 01:00:00 2
 [9,] 2001-01-02 11:00:00 2
[10,] 2001-01-03 21:00:00 2
> 
>     a[, mean(a), by = "date"]
           date  V1
[1,] 2001-01-01 1.5
[2,] 2001-01-02 1.4
[3,] 2001-01-03 2.0
> 
>     as.POSIXct(af$date, af$time) 
[1] "2001-01-01 00:00:00 EST" "2001-01-01 18:00:00 EST"
[3] "2001-01-02 12:00:00 EST" "2001-01-01 06:00:00 EST"
[5] "2001-01-02 00:00:00 EST" "2001-01-02 18:00:00 EST"
[7] "2001-01-01 12:00:00 EST" "2001-01-02 06:00:00 EST"
[9] "2001-01-03 00:00:00 EST"

- Tom

Tom Short
Electric Power Research Institute (EPRI)


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