chrono & time

Rust has two mature datetime ecosystems: chrono (widely used, feature-rich) and time (stdlib-aligned, conservative API). Pick one per workspace and serialize with serde.

Recipe

# Option A: chrono
chrono = { version = "0.4", features = ["serde"] }
 
# Option B: time
time = { version = "0.3", features = ["serde", "formatting", "parsing"] }

use chrono::{DateTime, Utc};
 
fn now_rfc3339() -> String {
    Utc::now().to_rfc3339()
}

use time::{OffsetDateTime, format_description::well_known::Rfc3339};
 
fn now_rfc3339_time() -> time::Result<String> {
    OffsetDateTime::now_utc().format(&Rfc3339)
}

When to reach for this:

API timestamps in JSON
Billing periods, TTLs, cron windows
Parsing user-facing date strings

Working Example

use chrono::{DateTime, Duration, Utc};
use serde::{Deserialize, Serialize};
 
#[derive(Serialize, Deserialize)]
struct Event {
    #[serde(with = "chrono::serde::ts_seconds")]
    created_at: DateTime<Utc>,
}
 
fn expires_in(event: &Event, days: i64) -> bool {
    Utc::now() > event.created_at + Duration::days(days)
}

What this demonstrates:

Store UTC in services; convert at UI edge
Serde adapters control wire format (epoch vs RFC3339)
Duration math for business rules

Deep Dive

Choosing chrono vs time

Factor	chrono	time
Ecosystem	sqlx, many crates default	Growing, `#![no_std]` friendly
API style	`DateTime<Utc>`	`OffsetDateTime`
Legacy code	Common in older repos	Preferred for new security-sensitive parsers

RFC3339 for APIs

Prefer ISO-8601 strings in JSON for human debugging. Use epoch seconds only when bandwidth matters.

Gotchas

Local timezone in servers - DST bugs. Fix: UTC internally; chrono-tz or time offset only at display.
Mixing chrono and time - conversion pain. Fix: workspace standard.
NaiveDateTime for instants - ambiguous. Fix: always attach offset or use UTC.
Parsing without validation - silent garbage. Fix: strict parsers and Result propagation.
Subsecond precision in DB - truncation. Fix: match column type (TIMESTAMPTZ).

Alternatives

Alternative	Use When	Don't Use When
`std::time::Instant`	Benchmarks, timeouts	User-visible dates
`humantime`	CLI duration strings	Typed domain models
DB `NOW()`	Persistence authority	Client-generated timestamps

FAQs

Is chrono deprecated?

Maintenance continues; time is an alternative, not a forced migration.

How do I serialize RFC3339?

chrono: to_rfc3339() or serde with chrono::serde::ts_seconds_option variants.

Timestamps in sqlx?

Use chrono::DateTime<Utc> or time::OffsetDateTime with compatible DB types.

Testing time-dependent code?

Inject a clock trait or pass DateTime<Utc> from caller; avoid hard-coded Utc::now() in core logic.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

chrono & time

Rust has two mature datetime ecosystems: chrono (widely used, feature-rich) and time (stdlib-aligned, conservative API). Pick one per workspace and serialize with serde.

Recipe

# Option A: chrono
chrono = { version = "0.4", features = ["serde"] }
 
# Option B: time
time = { version = "0.3", features = ["serde", "formatting", "parsing"] }

use chrono::{DateTime, Utc};
 
fn now_rfc3339() -> String {
    Utc::now().to_rfc3339()
}

use time::{OffsetDateTime, format_description::well_known::Rfc3339};
 
fn now_rfc3339_time() -> time::Result<String> {
    OffsetDateTime::now_utc().format(&Rfc3339)
}

When to reach for this:

API timestamps in JSON
Billing periods, TTLs, cron windows
Parsing user-facing date strings

Working Example

use chrono::{DateTime, Duration, Utc};
use serde::{Deserialize, Serialize};
 
#[derive(Serialize, Deserialize)]
struct Event {
    #[serde(with = "chrono::serde::ts_seconds")]
    created_at: DateTime<Utc>,
}
 
fn expires_in(event: &Event, days: i64) -> bool {
    Utc::now() > event.created_at + Duration::days(days)
}

What this demonstrates:

Store UTC in services; convert at UI edge
Serde adapters control wire format (epoch vs RFC3339)
Duration math for business rules

Deep Dive

Choosing chrono vs time

Factor	chrono	time
Ecosystem	sqlx, many crates default	Growing, `#![no_std]` friendly
API style	`DateTime<Utc>`	`OffsetDateTime`
Legacy code	Common in older repos	Preferred for new security-sensitive parsers

RFC3339 for APIs

Prefer ISO-8601 strings in JSON for human debugging. Use epoch seconds only when bandwidth matters.

Gotchas

Local timezone in servers - DST bugs. Fix: UTC internally; chrono-tz or time offset only at display.
Mixing chrono and time - conversion pain. Fix: workspace standard.
NaiveDateTime for instants - ambiguous. Fix: always attach offset or use UTC.
Parsing without validation - silent garbage. Fix: strict parsers and Result propagation.
Subsecond precision in DB - truncation. Fix: match column type (TIMESTAMPTZ).

Alternatives

Alternative	Use When	Don't Use When
`std::time::Instant`	Benchmarks, timeouts	User-visible dates
`humantime`	CLI duration strings	Typed domain models
DB `NOW()`	Persistence authority	Client-generated timestamps

FAQs

Is chrono deprecated?

Maintenance continues; time is an alternative, not a forced migration.

How do I serialize RFC3339?

chrono: to_rfc3339() or serde with chrono::serde::ts_seconds_option variants.

Timestamps in sqlx?

Use chrono::DateTime<Utc> or time::OffsetDateTime with compatible DB types.

Testing time-dependent code?

Inject a clock trait or pass DateTime<Utc> from caller; avoid hard-coded Utc::now() in core logic.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

chrono & time

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chrono & time