ww_date_time¶

ww_date_time gives device APIs a compact ISO 8601 date and time, nibble-packed instead of a fixed-width timestamp: DateTime, built from a NaiveDate, a NaiveTime, and an optional Timezone. It is no_std by default and depends only on shrink_wrap; an optional chrono feature adds conversions to and from the chrono crate's own date/time types for host-side code.

Add it to a device API crate with:

[dependencies]
ww_date_time = "0.1"

Types¶

• DateTime - date: NaiveDate, time: NaiveTime, timezone: Timezone; self_describing.
• NaiveDate - an ISO 8601 calendar date without timezone: year: Year, month: U4 (1-12), day: U5 (1-31); self_describing.
• Year - a signed year, -262_142..=262_141, stored as a UNib32 shifted by 2025 so that years close to now take the fewest nibbles; a year before 2025 is masked into the low 19 bits instead, so it always costs the same extra nibbles regardless of how far back it is (see sizes below). Year::new constructs one (bounds-checked), Year::year() reads it back as i32.
• NaiveTime - an ISO 8601 time without timezone: secs: U17 (seconds since midnight, 0-86399), frac: Option<U31> (sub-second part in nanoseconds, omitted entirely when zero); self_describing.
• Timezone - UTC (preferred, costs only 1 bit) or Other(OtherTimezone); self_describing, ww_repr = u1.
• OtherTimezone - Naive (no timezone information at all) or FixedOffset { secs: I18 } (offset from UTC in seconds); self_describing, ww_repr = u3 leaves room to add up to 6 more kinds later without breaking compatibility.

All of the above derive Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash (Timezone/OtherTimezone also Debug), and get owned (*Owned) counterparts when the std feature is enabled.

Sizes on the wire¶

DateTime's size depends on how far the year is from 2025 and on which fields are present, since Year, the Option<U31> fraction and the Timezone enum are all self-describing rather than fixed-width:

NaiveDate alone is 13 bits minimum (2025-2032), 17 bits for 2033-2088, and up to 37 bits for the widest years. NaiveTime alone is 18 bits without a fractional part, 49 bits with one.

A real DateTime for 2025-05-30 16:20:00 UTC (no fractional seconds) serializes to exactly 4 bytes (05 F3 96 C0), matching the 32-bit minimum above - ww_date_time's own test suite checks this against a real ser_shrink_wrap() call.

Construction¶

use ww_date_time::{DateTime, Timezone};

let utc = DateTime::from_ymd_hms_utc_opt(2025, 5, 30, 14, 0, 0, 0).unwrap();
assert_eq!(utc.timezone, Timezone::UTC);

let naive = DateTime::from_ymd_hms_naive_opt(2025, 5, 30, 14, 0, 0, 0).unwrap();

from_ymd_hms_utc_opt is a const fn (so it can build a DateTime in a const item, e.g. a compile-time build timestamp), from_ymd_hms_naive_opt is not, since it always picks Timezone::Other(OtherTimezone::Naive). Both return None on an out-of-range month, day, hour, minute, second or nanosecond, same as NaiveDate::from_ymd_opt and NaiveTime::from_hms_opt they build on.

Debug is implemented by hand for compact, human-readable output rather than deriving it: a NaiveDate prints as 2025-M5-D30, a NaiveTime as 14:0:0 (.123 appended when a fractional part is present), and DateTime as the two joined with the timezone.

chrono interop¶

With the chrono feature (on by default), ww_date_time re-exports the chrono crate and adds From/Into conversions in both directions: chrono::NaiveDate ↔ NaiveDate, chrono::NaiveTime ↔ NaiveTime, chrono::NaiveDateTime ↔ DateTime (as Timezone::Other(OtherTimezone::Naive)), and chrono::DateTime<Utc> / chrono::DateTime<FixedOffset> → DateTime. This is what lets host-side code build a DateTime from chrono::Utc::now() without touching the wire representation directly.

Features¶