penna 0.1.0

Structured JSON logging for tracing, in one line per event, without a regex engine underneath
Documentation 
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//! Timestamps, without a date crate.
//!
//! One format is needed — RFC 3339 in UTC with microseconds — and turning
//! seconds since the epoch into a civil date is thirty lines of arithmetic.

use std::fmt::Write as _;
use std::time::{SystemTime, UNIX_EPOCH};

/// Appends `YYYY-MM-DDTHH:MM:SS.ffffffZ`. A time before the epoch, which a
/// clock being set backwards can produce, is written as the epoch rather than
/// failing: a log line with an odd timestamp beats no log line.
pub(crate) fn write_rfc3339_micros(out: &mut String, at: SystemTime) {
    let since = at.duration_since(UNIX_EPOCH).unwrap_or_default();
    let seconds = since.as_secs();
    let micros = since.subsec_micros();
    let (year, month, day) = civil_from_days((seconds / 86_400) as i64);
    let remainder = seconds % 86_400;
    let _ = write!(
        out,
        "{year:04}-{month:02}-{day:02}T{:02}:{:02}:{:02}.{micros:06}Z",
        remainder / 3600,
        (remainder % 3600) / 60,
        remainder % 60
    );
}

/// Howard Hinnant's algorithm: days since 1970-01-01 to a proleptic Gregorian
/// date, exact for every day an `i64` can hold.
fn civil_from_days(days: i64) -> (i64, u32, u32) {
    let z = days + 719_468;
    let era = if z >= 0 { z } else { z - 146_096 } / 146_097;
    let day_of_era = z - era * 146_097;
    let year_of_era =
        (day_of_era - day_of_era / 1460 + day_of_era / 36_524 - day_of_era / 146_096) / 365;
    let year = year_of_era + era * 400;
    let day_of_year = day_of_era - (365 * year_of_era + year_of_era / 4 - year_of_era / 100);
    let month_index = (5 * day_of_year + 2) / 153;
    let day = (day_of_year - (153 * month_index + 2) / 5 + 1) as u32;
    let month = if month_index < 10 {
        month_index + 3
    } else {
        month_index - 9
    } as u32;
    (if month <= 2 { year + 1 } else { year }, month, day)
}

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

    fn at(seconds: u64, micros: u32) -> String {
        let mut out = String::new();
        write_rfc3339_micros(
            &mut out,
            UNIX_EPOCH + Duration::from_secs(seconds) + Duration::from_micros(micros as u64),
        );
        out
    }

    #[test]
    fn formats_the_epoch_leap_days_and_century_rules() {
        assert_eq!(at(0, 0), "1970-01-01T00:00:00.000000Z");
        assert_eq!(at(951_782_400, 0), "2000-02-29T00:00:00.000000Z");
        assert_eq!(at(4_107_542_399, 0), "2100-02-28T23:59:59.000000Z");
        assert_eq!(
            at(1_758_412_800 + 3_661, 991_167),
            "2025-09-21T01:01:01.991167Z"
        );
    }

    #[test]
    fn a_clock_before_the_epoch_does_not_panic() {
        let mut out = String::new();
        write_rfc3339_micros(&mut out, UNIX_EPOCH - Duration::from_secs(60));
        assert_eq!(out, "1970-01-01T00:00:00.000000Z");
    }
}