chrono 0.4.45: Disabling chrono default features strips Utc::now and Local which require the clock feature rather than std, while serde support requires an explicit opt-in serde feature and serializes DateTime as an RFC 3339 string

/target

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edition = "2024"

[dependencies]
chrono = { version = "0.4.45", default-features = false, features = ["alloc", "serde"] }
serde = { version = "1.0.229", features = ["derive"] }
serde_json = "1.0.151"

# CodeSampleX Notes: chrono feature gating with default-features = false

## Search Result
`search_known_solution` returned **MISS** (no match for `chrono` default features / feature gating; only unrelated generic serde and pillow samples were present).

## What a Model Would Have Written Instead
A naive model would configure `chrono = { version = "0.4", default-features = false, features = ["std"] }` expecting `Utc::now()` / `chrono::Local` to be available with `std`, and assume `DateTime<Utc>` automatically serializes with serde (or serializes to an integer Unix timestamp).

## How the Wrong Version Fails
- **Loud compile failure for clock**: Calling `Utc::now()` without `"clock"` fails with `error[E0599]: no associated function or constant named 'now' found for struct 'Utc'` (gated behind the `clock` feature, not `std`).
- **Loud compile failure for Local**: Importing `chrono::Local` fails with `error[E0432]: unresolved import 'chrono::Local'`.
- **Loud compile failure for Serde**: Omitting `features = ["serde"]` fails at compile-time with `error[E0277]: the trait bound 'DateTime<Utc>: serde::Serialize' is not satisfied` (serde is not in default features).
- **Format mismatch**: When enabled, `serde` serializes `DateTime<Utc>` into an RFC 3339 formatted string (e.g. `"2024-05-17T12:30:45Z"`), causing runtime deserialization failures if downstream systems expect numeric epoch timestamps.

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use chrono::{DateTime, Duration, NaiveDate, NaiveDateTime, NaiveTime, Utc};
use serde::{Deserialize, Serialize};

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct Record {
    pub id: u64,
    pub created_at: DateTime<Utc>,
    pub valid_date: NaiveDate,
}

pub fn make_deterministic_utc(
    year: i32,
    month: u32,
    day: u32,
    hour: u32,
    min: u32,
    sec: u32,
) -> Option<DateTime<Utc>> {
    let date = NaiveDate::from_ymd_opt(year, month, day)?;
    let time = NaiveTime::from_hms_opt(hour, min, sec)?;
    let naive_dt = NaiveDateTime::new(date, time);
    Some(DateTime::<Utc>::from_naive_utc_and_offset(naive_dt, Utc))
}

pub fn add_days(dt: DateTime<Utc>, days: i64) -> DateTime<Utc> {
    dt + Duration::days(days)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_serde_serializes_chrono_types_as_iso_strings() {
        let dt = make_deterministic_utc(2024, 5, 17, 12, 30, 45).expect("valid datetime");
        let record = Record {
            id: 42,
            created_at: dt,
            valid_date: NaiveDate::from_ymd_opt(2024, 5, 17).unwrap(),
        };

        let json_str = serde_json::to_string(&record).expect("serialization succeeds");

        // Chrono with serde feature serializes DateTime<Utc> to RFC 3339 string format,
        // not Unix epoch seconds and not a struct { secs, nanos }.
        assert!(
            json_str.contains(r#""created_at":"2024-05-17T12:30:45Z""#),
            "DateTime<Utc> must serialize to RFC 3339 string format, got: {json_str}"
        );
        assert!(
            json_str.contains(r#""valid_date":"2024-05-17""#),
            "NaiveDate must serialize to YYYY-MM-DD string format, got: {json_str}"
        );

        let deserialized: Record = serde_json::from_str(&json_str).expect("deserialization succeeds");
        assert_eq!(deserialized, record);
    }

    #[test]
    fn test_deserialization_rejects_invalid_calendar_dates() {
        // 2023 is not a leap year: Feb 29 must fail deserialization
        let invalid_leap_year_json = r#"{"id":1,"created_at":"2023-02-29T12:00:00Z","valid_date":"2023-02-29"}"#;
        let res = serde_json::from_str::<Record>(invalid_leap_year_json);
        assert!(res.is_err(), "Non-leap year Feb 29 must fail deserialization");

        // 2024 IS a leap year: Feb 29 must succeed deserialization
        let valid_leap_year_json = r#"{"id":1,"created_at":"2024-02-29T12:00:00Z","valid_date":"2024-02-29"}"#;
        let res: Result<Record, _> = serde_json::from_str(valid_leap_year_json);
        assert!(res.is_ok(), "Leap year Feb 29 must succeed deserialization");
    }

    #[test]
    fn test_pure_arithmetic_without_system_clock() {
        let base = make_deterministic_utc(2024, 2, 28, 23, 0, 0).expect("valid datetime");
        let next_day = add_days(base, 1);

        // Leap year addition rolls over to 2024-02-29
        assert_eq!(next_day.to_rfc3339(), "2024-02-29T23:00:00+00:00");
    }

    #[test]
    fn test_rfc3339_parsing_and_formatting() {
        let parsed: DateTime<Utc> = DateTime::parse_from_rfc3339("2024-12-31T23:59:59Z")
            .expect("parse valid RFC 3339")
            .with_timezone(&Utc);

        assert_eq!(parsed.timestamp(), 1735689599);
    }
}