Leap year and last day of month

[gwenn]

Would you mind adding something like:

fn is_leap_year(year: i32) -> bool {
    return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)
}

fn last_day_of_month(year: i32, month: u32) -> u32 {
    match month {
        1 | 3 | 5 | 7 | 8 | 10 | 12 => 31,
        4 | 6 | 9 | 11 => 30,
        2 => if is_leap_year(year) { 29 } else { 28 },
        _ => panic!("invalid month: {}" , month),
    }
}

?
Thanks.

[lifthrasiir]

While Chrono is inherently tied to the proleptic Gregorian calendar, I don't think such specific functions are not a good fit for general purpose library. If you only care about the correct answer (and not the performance), the following can be used:

fn is_leap_year(year: i32) -> bool {
    NaiveDate::from_ymd_opt(year, 2, 29).is_some()
}

fn last_day_of_month(year: i32, month: 32) -> u32 {
    NaiveDate::from_ymd_opt(year, month + 1, 1).unwrap_or(NaiveDate::from_ymd(year + 1, 1, 1)).pred().day()
}

I think JodaTime does have methods like is_leap, but mostly for supporting different calendar systems with varying leap definitions.

[westy92]

You can add this functionality for your project with:

trait NaiveDateExt {
    fn days_in_month(&self) -> i32;
    fn days_in_year(&self) -> i32;
    fn is_leap_year(&self) -> bool;
}

impl NaiveDateExt for chrono::NaiveDate {
    fn days_in_month(&self) -> i32 {
        let month = self.month();
        match month {
            1 | 3 | 5 | 7 | 8 | 10 | 12 => 31,
            4 | 6 | 9 | 11 => 30,
            2 => if self.is_leap_year() { 29 } else { 28 },
            _ => panic!("Invalid month: {}" , month),
        }
    }

    fn days_in_year(&self) -> i32 {
        if self.is_leap_year() { 366 } else { 365 }
    }

    fn is_leap_year(&self) -> bool {
        let year = self.year();
        return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)
    }
}

[sshine]

such specific functions are not a good fit for general purpose library

I apologize for necroposting, but could you explain why?

A safe variation of the last_day_of_month() posted earlier,

pub fn last_of_month(year: i32, month: u32) -> Option<chrono::NaiveDate> {
    chrono::NaiveDate::from_ymd_opt(year, month + 1, 1)
        .or_else(|| chrono::NaiveDate::from_ymd_opt(year + 1, 1, 1))?
        .pred_opt()
}

[pitdicker]

I have seen this request more than once, and I also think it is reasonable and useful to add.

#1247 includes NaiveDate::leap_year, and #69 is the open issue for adding something like last_day_of_month(). I used the same workaround as you in a number of cases.

I am not sure yet about the best API for something like last_day_of_month(). In one case (the NaiveDate::diff_months_days() method in #1247) it was not enough for me to know the number of days in the current month, it needed the number in the previous month. #69 (comment) suggests adding a type such as YearMonth and implementing the method on that would be more flexible, and #203 also requests such a type.