R/yrfraction.R

In season: Seasonal analysis of health data

# yrfraction.R
# fraction of the year for a date, includes leap year
# type = 'monthly' or 'daily' (default)
# April 2009

yrfraction<-function(date,type='daily'){
  if (type=='daily'){
    if (class(date)!="Date"){stop("Date variable for annual data must be in date format, see ?Dates")} 
    year<-as.numeric(format(date,'%Y'));
    lastday<-ISOdate(year,12,31); # last day in December
    day<-as.numeric(format(date,'%j')); # Day of year as decimal number (001-366)
    yrlength<-as.numeric(format(lastday,'%j'));
    yrfrac<-(day-1)/yrlength;
  }
  if (type=='monthly'){
    if (max(date)>12|min(date)<1){stop("Date variable for monthly data must be month integer (1 to 12)")} 
    yrfrac<-(date-1)/12;
  }
  return(yrfrac)
}