cal.yr: Functions to convert character, factor and various date objects into a number, and vice versa.

Description

Dates are converted to a numerical value, giving the calendar year as a fractional number. 1 January 1970 is converted to 1970.0, and other dates are converted by assuming that years are all 365.25 days long, so inaccuracies may arise, for example, 1 Jan 2000 is converted to 1999.999. Differences between converted values will be 1/365.25 of the difference between corresponding Date objects.

Usage

cal.yr( x, format="%Y-%m-%d", wh=NULL )
  # S3 method for cal.yr
as.Date( x, ... )

Value

cal.yr returns a numerical vector of the same length as

x, of class c("cal.yr","numeric"). If x is a data frame a dataframe with some of the columns converted to class "cal.yr" is returned.

as.Date.cal.yr returns a Date object.

Arguments

x: A factor or character vector, representing a date in format format, or an object of class Date, POSIXlt, POSIXct, date, dates or chron (the latter two requires the chron package). If x is a data frame, all variables in the data-frame which are of one the classes mentioned are converted to class cal.yr. See arguemt wh, though.
format: Format of the date values if x is factor or character. If this argument is supplied and x is a datafame, all character variables are converted to class cal.yr. Factors in the dataframe will be ignored.
wh: Indices of the variables to convert if x is a data frame. Can be either a numerical or character vector.
...: Arguments passed on from other methods.

Author

Bendix Carstensen, Steno Diabetes Center Copenhagen, b@bxc.dk, http://bendixcarstensen.com

Examples

Run this code

 # Character vector of dates:
 birth <- c("14/07/1852","01/04/1954","10/06/1987","16/05/1990",
            "12/11/1980","01/01/1997","01/01/1998","01/01/1999")
 # Proper conversion to class "Date":
 birth.dat <- as.Date( birth, format="%d/%m/%Y" )
 # Converson of character to class "cal.yr"
 bt.yr <- cal.yr( birth, format="%d/%m/%Y" )
 # Back to class "Date":
 bt.dat <- as.Date( bt.yr )
 # Numerical calculation of days since 1.1.1970:
 days <- Days <- (bt.yr-1970)*365.25
 # Blunt assignment of class:
 class( Days ) <- "Date"
 # Then data.frame() to get readable output of results:
 data.frame( birth, birth.dat, bt.yr, bt.dat, days, Days, round(Days) )