Epi (version 0.2)

Calendar year: Functions representing dates as fractional calendar years

Description

Dates are converted to a numerical value, so that 1 January 1970 is converted to 1970.0. The result is given a class attribute, cal.yr. The function is intended for epidemiological use, and the dates are converted to a calendar time scale that has 4 years of equal length between 1.1.1970 and 1.1.1974 etc. The printing of cal.yr objects in human readable form may therefore be as much as 3 days wrong.

Usage

cal.yr(x, format)
  dmy.cal.yr( d, m, y )
  mdy.cal.yr( m, d, y )
  ymd.cal.yr( y, m, d )
  weekdays.cal.yr( x, abbreviate=FALSE )
  weeks( x )
  weeks.cal.yr( x )
  months.cal.yr( x, abbreviate=FALSE )
  print.cal.yr( x, format="%d/%m/%Y", ... )

Arguments

x

A character vector, representing a date in format format, or an object of class POSIXt or Date.

d, m, y

Numerical vectors representing, date month and year.

format

Optional character values giving the format for the date if given as character. See the help page for strptime for admissible values.

abbreviate

Should names be abbreviated?

...

Ignored, but present for compatibility with print methods

Value

• cal.yr returns a numerical vector of the same length as x. The associated print method, print.cal.yr prints the dates in human readable format. To see the dates as fractions of years, use as.numeric(cy). dmy.cal.yr, mdy.cal.yr and ymd.cal.yr are convenience functions for converting separate numerical variables with date d, month m and year y into dates of class cal.yr. These functions also accept a single character variable as input, with two digits for day and month and four for year and any delimiters. weekdays.cal.yr, weeks.cal.yr and months.cal.yr, extract weekday, week number and month from calendar year objects. weekdays and months return factors with 7 and 12 levels respectively, regardless of the actually occurring number of levels. weeks return numeric values in the range 1--52.

See Also

DateTimeClasses, Date

Examples

birth <- c("14/07/1952", "01/04/1954", "10/06/1987")
 ( bt.yr <- cal.yr( birth, format="%d/%m/%Y" ) )
 as.numeric( bt.yr )
 weeks( bt.yr )
 months( bt.yr )
 ( bt.y <- dmy.cal.yr( birth ) )