age_calc in eeptools producing calculation error

Question

I am trying to calculate age in months and one particular date is giving me an incorrect result

See code below. For some reason when the date is March 1, 2019 the age in months is 2.9, which is incorrect, but for March 2, 2019 the function works better although I believe the result should be 4.x.

age_calc(as.Date("10/30/18",format="%m/%d/%y"),as.Date("3/1/19",format="%m/%d/%y"),units="months")
#[1] 2.960829

age_calc(as.Date("10/30/18",format="%m/%d/%y"),as.Date("3/2/19",format="%m/%d/%y"),units="months")
#[1] 3.993088

is this an error in the function? Or I am I doing something wrong? Is this an issue as February has fewer days??

Could you specify what package the `age_calc` function is a part of? Or provide documentation to that function? — Felix T., Jun 06 '19 at 22:18
@FelixT. `age_calc` is from `eeptools` (says so in the title) — Maurits Evers, Jun 06 '19 at 22:21
https://stackoverflow.com/questions/3611314/calculate-ages-in-r This helped me a lot! — Ryan John, Jun 06 '19 at 22:24

score 1 · Accepted Answer · answered Jun 06 '19 at 22:18

This is perhaps more of an extended comment.

eeptools::age_calc seems to calculate the age in months in an unconventional way (you can see the source code when you type age_calc into an R terminal and hit Enter).

Perhaps a more canonical way to calculate the age between two dates in months is to simply divide the interval by the unit duration. A relevant & interesting post is Get the difference between dates in terms of weeks, months, quarters, and years.

From said post, @Gregor defined a convenient function that does something similar to eeptools::age_calc

library(lubridate)
age <- function(dob, age.day = today(), units = "years", floor = TRUE) {
    calc.age = interval(dob, age.day) / duration(num = 1, units = units)
    if (floor) return(as.integer(floor(calc.age)))
    return(calc.age)
}

Using age we then get

age(
    as.Date("10/30/18",format="%m/%d/%y"),
    as.Date("3/1/19",format="%m/%d/%y"),
    units="months", floor = FALSE)
#[1] 4.010959

age(
    as.Date("10/30/18",format="%m/%d/%y"),
    as.Date("3/2/19",format="%m/%d/%y"),
    units="months", floor = FALSE)
#[1] 4.043836

These values are consistent with values that e.g. Wolfram Alpha gives for the same dates.

age_calc in eeptools producing calculation error

1 Answers1