Function add_date_time_diff 

pub fn add_date_time_diff<Tz: TimeZone>(
    from: DateTime<Tz>,
    date_time_diff: &dyn DateTimeDiff,
) -> LocalResult<DateTime<Tz>>

Calculate from + date_time_diff.

Example

use chrono::prelude::*;
use date_differencer::{add_date_time_diff, DateDiffResult};

let date = Local.with_ymd_and_hms(2000, 1, 1, 0, 0, 0).unwrap();

let date_after_1_year_1_day = add_date_time_diff(date, &DateDiffResult {
    years: 1,
    days: 1,
    ..DateDiffResult::default()
})
.unwrap();

assert_eq!(
    Local.with_ymd_and_hms(2001, 1, 2, 0, 0, 0).unwrap(),
    date_after_1_year_1_day
)