doc(sqlite): document behavior for zoned date-time types (#3493)

sqlx-sqlite/src/types/mod.rs

@@ -65,7 +65,36 @@
 //!   November 24, 4714 BCE 12:00 UTC, as if returned from the `julianday()` function.
 //!
 //! These types will always encode to a datetime string, either
-//! with (`DateTime<Tz>` for any `Tz: TimeZone`) or without (`NaiveDateTime`) a timezone offset.
+//! with a timezone offset (`DateTime<Tz>` for any `Tz: TimeZone`) or without (`NaiveDateTime`).
+//!
+//! ##### NOTE: `CURRENT_TIMESTAMP` and comparison/interoperability of `DATETIME` values
+//! As stated previously, `DateTime<Tz>` always encodes to a date-time string
+//! _with_ a timezone offset,
+//! in [RFC 3339 format][::chrono::DateTime::to_rfc3339_opts] (with `use_z: false`).
+//!
+//! However, most of SQLite's datetime functions
+//! (including `datetime()` and `DEFAULT CURRENT_TIMESTAMP`)
+//! do not use this format. They instead use `YYYY-MM-DD HH:MM:SS.SSSS` without a timezone offset.
+//!
+//! This may cause problems with interoperability with other applications, and especially
+//! when comparing datetime values, which compares the actual string values lexicographically.
+//!
+//! Date-time strings in the SQLite format will generally _not_ compare consistently
+//! with date-time strings in the RFC 3339 format.
+//!
+//! We recommend that you decide up-front whether `DATETIME` values should be stored
+//! with explicit time zones or not, and use the corresponding type
+//! (and its corresponding offset, if applicable) _consistently_ throughout your
+//! application:
+//!
+//! * RFC 3339 format: `DateTime<Tz>` (e.g. `DateTime<Utc>`, `DateTime<Local>`, `DateTime<FixedOffset>`)
+//!   * Changing or mixing and matching offsets may break comparisons with existing timestamps.
+//!   * `DateTime<Local>` is **not recommended** for portable applications.
+//!   * `DateTime<FixedOffset>` is only recommended if the offset is **constant**.
+//! * SQLite format: `NaiveDateTime`
+//!
+//! Note that non-constant offsets may still cause issues when comparing timestamps,
+//! as the comparison operators are not timezone-aware.
 //!
 //! ### [`time`](https://crates.io/crates/time)
 //!
@@ -82,6 +111,37 @@
 //! The behavior here is identical to the corresponding `chrono` types, minus the support for `REAL`
 //! values as Julian days (it's just not implemented).
 //!
+//! `PrimitiveDateTime` and `OffsetDateTime` will always encode to a datetime string, either
+//! with a timezone offset (`OffsetDateTime`) or without (`PrimitiveDateTime`).
+//!
+//! ##### NOTE: `CURRENT_TIMESTAMP` and comparison/interoperability of `DATETIME` values
+//! As stated previously, `OffsetDateTime` always encodes to a datetime string _with_ a timezone offset,
+//! in [RFC 3339 format][::time::format_description::well_known::Rfc3339] (using `Z` for UTC offsets).
+//!
+//! However, most of SQLite's datetime functions
+//! (including `datetime()` and `DEFAULT CURRENT_TIMESTAMP`)
+//! do not use this format. They instead use `YYYY-MM-DD HH:MM:SS.SSSS` without a timezone offset.
+//!
+//! This may cause problems with interoperability with other applications, and especially
+//! when comparing datetime values, which compares the actual string values lexicographically.
+//!
+//! Date-time strings in the SQLite format will generally _not_ compare consistently
+//! with date-time strings in the RFC 3339 format.
+//!
+//! We recommend that you decide up-front whether `DATETIME` values should be stored
+//! with explicit time zones or not, and use the corresponding type
+//! (and its corresponding offset, if applicable) _consistently_ throughout your
+//! application:
+//!
+//! * RFC 3339 format: `OffsetDateTime` with a **constant** offset.
+//!   * Changing or mixing and matching offsets may break comparisons with existing timestamps.
+//!   * `OffsetDateTime::now_local()` is **not recommended** for portable applications.
+//!   * Non-UTC offsets are only recommended if the offset is **constant**.
+//! * SQLite format: `PrimitiveDateTime`
+//!
+//! Note that non-constant offsets may still cause issues when comparing timestamps,
+//! as the comparison operators are not timezone-aware.
+//!
 //! ### [`uuid`](https://crates.io/crates/uuid)
 //!
 //! Requires the `uuid` Cargo feature flag.