rust chrono

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use chrono::prelude::*;   // 1. 时间转字符 // 2. 字符转时间 // 3. 时间相加减 // 4. 时间加差异数   fn main(){     let d = NaiveDate::from_ymd(2015, 3, 14);     let a = NaiveDate::from_ymd(2015, 3, 16);     let a_b :i64 = (d-a).num_days();     // println!("{:?}", (d-a).num_days());     // let c = a + chrono::Duration::from(1);     // println!("{:?}", c);       let x = NaiveDate::parse_from_str("2015/9/30", "%Y/%m/%d");     // println!("{:?}",x);       let y = x.unwrap() + chrono::Duration::days(-1_i64);　　//i64类型     print!(" => {:?}", y); }