chrono 0.4.39: Construct bounded timezone offsets with FixedOffset, preserve Unix timestamps across timezone conversions, and format offsets with strftime colons specifiers

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[package]
name = "chrono-fixed-offset-depth"
version = "0.1.0"
edition = "2021"

[dependencies]
chrono = "=0.4.39"

# Chrono FixedOffset and Timezone Conversion Semantics

## Search Result
The known-solution search returned three existing samples:
- `sha256:861889cfdd0a5dd5ceeb51b6ec6359baf342d7c5c30f7d1fbc598a53e175fac2`: Disabling chrono default features and serde formatting.
- `sha256:16f5b2fe847fd1153f4755c8bfea4f30e6e6f6ed5be62408dbac23c8eb592e44`: Parsing fixed-length fractional seconds with trailing literals in chrono 0.4.42.
- `sha256:bb53a07e8a52f98a3d5f6b3a0c79b1070e860b17949e852a6e2f2c015657ec3f`: Non-deprecated constructors and month arithmetic clamping.

This sample focuses on a different area of the library: `FixedOffset` boundary constraints, timezone conversion invariance, and strftime offset formatting specifiers.

## Pinned Release
- Package: `chrono`
- Version: `0.4.39`

## Observed Behaviour

1. **Offset Bounds Validation**:
   - `FixedOffset::east_opt(seconds)` and `FixedOffset::west_opt(seconds)` enforce a strict range of `-86400 < seconds < 86400` (strictly less than 24 hours).
   - Passing `86_400` or `-86_400` returns `None`, whereas `86_399` and `-86_399` return `Some(FixedOffset)`.
   - `FixedOffset::east_opt(-18000)` produces the same offset as `FixedOffset::west_opt(18000)`, with `local_minus_utc()` returning `-18000`.

2. **Timezone Conversion and Instant Invariance**:
   - Calling `dt.with_timezone(&target_offset)` on `DateTime<FixedOffset>` shifts the local calendar date and time representation (e.g. `2024-07-15 08:30:00 -05:00` becomes `2024-07-15 19:00:00 +05:30`).
   - The underlying Unix epoch timestamp (`dt.timestamp()`) is invariant across conversions.

3. **Strftime Offset Formatting Specifiers**:
   - `%z` formats the offset without separators: `+0530`.
   - `%:z` formats the offset with colon-separated minutes: `+05:30`.
   - `%::z` formats the offset with seconds included: `+05:30:00`.
   - `%:::z` formats the offset with hour precision only: `+05`.
   - Parsing strings via `DateTime::parse_from_str` with `%z` or `%:z` correctly constructs the `DateTime<FixedOffset>` instance.

4. **Duration Arithmetic**:
   - Adding a `Duration` to `DateTime<FixedOffset>` increments both the local wall clock time and the Unix epoch timestamp by the duration amount while preserving the timezone offset unchanged.

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use chrono::{DateTime, Duration, FixedOffset};

/// Constructs a FixedOffset from east offset in seconds, returning None if out of range (-86400..86400).
pub fn make_east_offset(seconds: i32) -> Option<FixedOffset> {
    FixedOffset::east_opt(seconds)
}

/// Constructs a FixedOffset from west offset in seconds, returning None if out of range (-86400..86400).
pub fn make_west_offset(seconds: i32) -> Option<FixedOffset> {
    FixedOffset::west_opt(seconds)
}

/// Converts a DateTime<FixedOffset> to another timezone offset, preserving the UTC instant.
pub fn convert_offset(
    dt: DateTime<FixedOffset>,
    target: FixedOffset,
) -> DateTime<FixedOffset> {
    dt.with_timezone(&target)
}

/// Formats a DateTime<FixedOffset> with custom format specifiers.
pub fn format_datetime(dt: &DateTime<FixedOffset>, fmt: &str) -> String {
    dt.format(fmt).to_string()
}

/// Adds a duration to DateTime<FixedOffset>, shifting the UTC instant and wall clock while keeping offset unchanged.
pub fn add_duration(dt: DateTime<FixedOffset>, duration: Duration) -> DateTime<FixedOffset> {
    dt + duration
}

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

    #[test]
    fn test_fixed_offset_bounds_and_symmetry() {
        // Offsets must be strictly within -86_400..86_400 seconds (exclusive of 24h boundary)
        assert!(make_east_offset(86_400).is_none());
        assert!(make_east_offset(-86_400).is_none());
        assert!(make_west_offset(86_400).is_none());
        assert!(make_west_offset(-86_400).is_none());

        assert!(make_east_offset(86_399).is_some());
        assert!(make_west_offset(86_399).is_some());

        // Negative east offset is identical to positive west offset
        let east_neg = make_east_offset(-18000).unwrap();
        let west_pos = make_west_offset(18000).unwrap();
        assert_eq!(east_neg, west_pos);
        assert_eq!(east_neg.local_minus_utc(), -18000);
    }

    #[test]
    fn test_timezone_conversion_preserves_timestamp() {
        let dt = DateTime::parse_from_rfc3339("2024-07-15T08:30:00-05:00")
            .expect("valid RFC 3339 string");
        assert_eq!(dt.timestamp(), 1721050200);

        let ist = make_east_offset(5 * 3600 + 1800).unwrap();
        let dt_ist = convert_offset(dt, ist);

        // Instant (epoch timestamp) is invariant under timezone conversion
        assert_eq!(dt_ist.timestamp(), dt.timestamp());
        assert_eq!(
            format_datetime(&dt_ist, "%Y-%m-%dT%H:%M:%S%:z"),
            "2024-07-15T19:00:00+05:30"
        );

        // Converting to Utc also preserves epoch timestamp
        let dt_utc = dt.with_timezone(&Utc);
        assert_eq!(dt_utc.timestamp(), dt.timestamp());
        assert_eq!(dt_utc.to_rfc3339(), "2024-07-15T13:30:00+00:00");
    }

    #[test]
    fn test_strftime_offset_specifiers_and_parsing() {
        let ist = make_east_offset(5 * 3600 + 1800).unwrap();
        let naive = chrono::NaiveDate::from_ymd_opt(2024, 7, 15)
            .unwrap()
            .and_hms_opt(19, 0, 0)
            .unwrap();
        let dt = DateTime::<FixedOffset>::from_naive_utc_and_offset(
            naive - Duration::seconds(5 * 3600 + 1800),
            ist,
        );

        // Offset format specifiers: %z (+HHMM), %:z (+HH:MM), %::z (+HH:MM:SS), %:::z (+HH)
        assert_eq!(format_datetime(&dt, "%z"), "+0530");
        assert_eq!(format_datetime(&dt, "%:z"), "+05:30");
        assert_eq!(format_datetime(&dt, "%::z"), "+05:30:00");
        assert_eq!(format_datetime(&dt, "%:::z"), "+05");

        // Parsing from strings with compact (%z) and colon-separated (%:z) timezone offsets
        let p1 = DateTime::parse_from_str("2024-07-15 19:00:00 +0530", "%Y-%m-%d %H:%M:%S %z")
            .expect("parse with %z");
        let p2 = DateTime::parse_from_str("2024-07-15 19:00:00 +05:30", "%Y-%m-%d %H:%M:%S %:z")
            .expect("parse with %:z");

        assert_eq!(p1, dt);
        assert_eq!(p2, dt);
    }

    #[test]
    fn test_duration_arithmetic_on_offset_datetime() {
        let dt = DateTime::parse_from_rfc3339("2024-07-15T08:30:00-05:00")
            .expect("valid RFC 3339 string");
        let advanced = add_duration(dt, Duration::hours(3));

        // Offset is preserved, timestamp and local wall clock advance together
        assert_eq!(advanced.offset(), dt.offset());
        assert_eq!(advanced.timestamp(), dt.timestamp() + 3 * 3600);
        assert_eq!(
            format_datetime(&advanced, "%Y-%m-%dT%H:%M:%S%:z"),
            "2024-07-15T11:30:00-05:00"
        );
    }
}