# DateStyle

> DateStyle is the PostgreSQL setting that defines the display format for date and time values.
---

> [!NOTE]
> **Fact — official short description:** “Sets the display format for date and time values.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `string` | Upstream pg_settings type |
| Context | `user` | Settable by an ordinary user |
| Unit | — | Raw unit |
| Range | — | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Client Connection Defaults / Locale and Formatting | Upstream classification |
| Latest boot value | `ISO, MDY` | ISO, MDY |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG9.0 (research boundary) |
| Present in | PG9.0–19 Beta 3 |
| Removed in | No |
| Introduction commit | Not asserted: predates the PG9.0 research boundary |
| Commit date | — |
| Discussion | — |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG9.0–19 Beta 3 | `ISO, MDY` | — | ISO, MDY |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

DateStyle sets the display format for date and time values. Also controls interpretation of ambiguous date inputs. The value has two independent parts: an output style and a DMY/MDY/YMD field order; the latter also resolves ambiguous input.

DateStyle is a USER-context setting. An authorized role can change it for a session, while ALTER ROLE or ALTER DATABASE can establish a default for future sessions.

It changes representation, parsing, or locale behavior at the client boundary rather than physical storage. Coordinate it with the other locale and formatting settings and with driver-native binary or typed protocols.

## Tuning advice {#tuning-advice}

> [!TIP]
> **Advice.** These are workload-specific starting points and must be validated with measurements.

| Workload | Guidance |
| --- | --- |
| OLTP | Treat DateStyle as an application contract, not a performance knob. Standardize it per role or connection pool and keep wire formats explicit where clients parse text. |
| OLAP | Pin DateStyle for export, reporting, and reproducible analytical jobs; prefer explicit SQL formatting when a file or API has a durable schema. |
| Small nodes | Keep the upstream or locale-derived value unless a client requires another one. A smaller server gains no capacity from changing DateStyle. |
{.full-width}

## Pigsty {#pigsty}

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG19 Beta 3; this does not assert current Pigsty support for that historical or beta release.

| Template | Effective value | Versus upstream boot | Source expression |
| --- | --- | --- | --- |
| OLTP | Unmodified | — | — |
| OLAP | Unmodified | — | — |
| CRIT | Unmodified | — | — |
| TINY | Unmodified | — | — |
{.full-width}

> [!CAUTION]
> **Advice — pending human review.** Fact from the current Pigsty template projection: OLTP: PG9.0–19 Beta 3 unmodified; OLAP: PG9.0–19 Beta 3 unmodified; CRIT: PG9.0–19 Beta 3 unmodified; TINY: PG9.0–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Changing DateStyle in one session and assuming role defaults, database defaults, or other pooled sessions changed with it.
- Assuming a presentation setting changes stored values or server-side type semantics.
- Changing a role or database default without testing text-parsing clients, exports, and pooled sessions.
- Parsing an ambiguous value such as 01/02/03 under an unexpected MDY/DMY order.

## Related parameters {#related-parameters}

[`IntervalStyle`](/parameters/intervalstyle/) · [`TimeZone`](/parameters/timezone/) · [`lc_time`](/parameters/lc-time/) · [`timezone_abbreviations`](/parameters/timezone-abbreviations/) · [`log_timezone`](/parameters/log-timezone/)

## References {#references}

- [PostgreSQL 19 Beta 3 — DateStyle](https://www.postgresql.org/docs/19/runtime-config-client.html#GUC-DATESTYLE)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)