How to get the difference between two chrono dates? - date

Question

I'd like to get a "time-span" difference between today 08/11/2020 and 01/01/1970 in such format:
Timespan:
50 year(s) 10 month(s) 7 day(s)
The code below correctly calculates this for: today - 01/01/1970. However, it fails miserably for input like 23/12/1970 giving:
50 year(s) -1 month(s) -15 day(s)
Whereas the expected result is 49 year(s) 10 month(s) 16 day(s).
More dates to test:
09/01/1985
Got: 35 year(s) 10 month(s) -1 day(s) but expected 35 year(s) 9 month(s) 30 day(s).
24/02/1936
Got: 84 year(s) 9 month(s) -16 day(s) but expected 84 year(s) 8 month(s) 15 day(s).
Cargo.toml:
[package]
name = "relative-delta"
version = "0.1.0"
authors = ["baduker"]
edition = "2018"
[dependencies]
chrono = "0.4.19"
Code:
use chrono::{NaiveDateTime, DateTime, Utc, Datelike};
fn main() {
let date_string = "23/12/1970";
let naive_date = chrono::NaiveDate::parse_from_str(date_string, "%d/%m/%Y").unwrap();
let naive_datetime: NaiveDateTime = naive_date.and_hms(0, 0, 0);
let date = DateTime::<Utc>::from_utc(naive_datetime, Utc);
let years = Utc::now().year() - date.year();
let months = Utc::now().month() as i64 - date.month() as i64;
let days = Utc::now().day() as i64 - date.day() as i64;
println!("Timespan:\n{} year(s) {} month(s) {} day(s)", years, months, days);
}

Answer 1

Your calculation does not make sense to me e.g. if today's date is 1st Nov 2021, your calculation would return 1 - 23 = -22 as the number of days. Today is my third day of learning Rust and therefore I do not have enough expertise to give you an optimized solution. However, if you assume a year of 365 days and a month of 30 days, the following solution should meet your requirement.
use chrono::{DateTime, Utc};
fn main() {
// Tests
print_diff("23/12/1970");
}
fn print_diff(date_string: &str) {
let today = Utc::now();
let datetime =
DateTime::<Utc>::from_utc(chrono::NaiveDate::parse_from_str(date_string, "%d/%m/%Y")
.unwrap()
.and_hms(0, 0, 0), Utc);
let diff = today.signed_duration_since(datetime);
let days = diff.num_days();
let years = days / 365;
let remaining_days = days % 365;
let months = remaining_days / 30;
let days = remaining_days % 30;
println!("{} years {} months {} days", years, months, days);
}
Output:
50 years 11 months 9 days
Playground