Datelike should have methods to add/sub a number of calendar day/month/year

[vincentdephily]

Datelike currently offers a set of with_*() methods to set the day/month/year, but the resulting date must be valid (for example calling with_month(13) doesn't translate to january of the next year). You can also add a Duration to a Datelike, but there's no good Duration value for year/month/day because of leap years, various month lengths, and DST.

I'd like to have methods add_days/month/years(i32) -> Self to easily add/subtract the corresponding number of time units. Might want to have add_hour/min/sec/etc() for consistency's sake, although adding a Duration works well in this case (unless we want to handle DST ?).

My usecase is deceptively simple: I write a report and want to know when the next day/week/month/year starts, after a given timestamp. For example I have 2018-11-14T12:34:56z as input and want 2018-12-14T00:00:00z as output (beginning of next month).

This is ugly but doable for "next year":

fn next_year<T>(d: T) -> T where T: Datelike {
  d.with_year(d.year()+1)
    .unwrap()
   .with_month(1)
   .unwrap()
   .with_day(1)
   .unwrap()
   .with_hour(0)
   .unwrap()
   .with_minute(0)
   .unwrap()
   .with_second(0)
   .unwrap()
   // Thankfully I know that my input had a second granularity so I don't care about nanoseconds.
}

But the implementation for next_month/week/day() is not going to be fun.

Incidentally, it with also be nice to have a with_year/month/day/etc() that panics instead of returning a result, for cases like this where the input is hardcoded and known to be correct. Or a with_ymd() and with_hmsn() ;)

[felipenoris]

I have a similar issue. I just want to get the next/last day out of a Datelike date. For now I'm doing this:

fn next_date<T: Datelike + Copy>(date: T, fwd: bool) -> T {
    let inc: i32 = {
        if fwd {
            1
        } else {
            -1
        }
    };

    match date.with_ordinal((date.ordinal() as i32 + inc) as u32) {
        Some(dt) => dt,
        None =>  {
            if fwd {
                date
                    .with_year(date.year() + 1).unwrap()
                    .with_ordinal(1).unwrap()
            } else {
                date
                    .with_year(date.year() - 1).unwrap()
                    .with_month(12).unwrap()
                    .with_day(31).unwrap()
            }
        }
    }
}

[nyovaya]

I would like to substract NaiveDate from DateTime.

[pitdicker]

Fixed with #784 and #731.