calc_age.r

# Useful lubridate function for date subtraction written by mmparker

# Calculate age at a given reference date
# Create an interval between the date of birth and the enrollment date; 
# intervals are specific to the two dates. Periods give the actual length
# of time between those dates, so convert to period and extract the year.

calc_age <- function(birthDate, refDate = Sys.Date()) {

    require(lubridate)

    period <- as.period(new_interval(birthDate, refDate),
                        unit = "year")

    period$year

}


# Examples
calc_age("1990-06-30")                 # As long as the date is %Y-%m-%d formatted, it can be a character
calc_age("1990-06-30", "2003-07-12")   # Calculate age at any date

# Works for multiple reference dates, too
calc_age("1990-06-30", seq(from = as.Date("2003-01-01"), to = as.Date("2012-01-01"), by = "year"))