---
title: "day"
slug: "dev-database-reference-sql-functions-date-time-day"
updated: 2026-10-30T18:19:01Z
published: 2026-10-30T18:19:01Z
canonical: "docs.vastdata.com/dev-database-reference-sql-functions-date-time-day"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vastdata.com/llms.txt
> Use this file to discover all available pages before exploring further.

# day

*Applies to VAST Cluster 5.5.*

*[Functions](/develop/docs/dev-database-reference-sql-functions-index) › [Date and time functions](/develop/docs/dev-database-reference-sql-functions-date-time) › Parts of a date or time.* Returns the day of the month of a date or timestamp, from 1 to 31, or the days field of an interval. `dayofmonth` is the same function.

## Syntax

```
day(value)
dayofmonth(value)
```

| Argument | Type | Description |
| --- | --- | --- |
| `value` | `DATE`, `TIMESTAMP` (also `TIMESTAMP_NS`, `TIMESTAMP_MS`, `TIMESTAMP_S`) or `INTERVAL` | The value to take the day of. |

**Returns:** `BIGINT`.

**Aliases:** `dayofmonth`. Also the same as `date_part('day', value)` and `extract(day FROM value)`; see [date_part](/develop/docs/dev-database-reference-sql-functions-date-time-date-part).

## Set up the examples

The examples connect with the ADBC driver, as in [VAST Query Engine (ADBC)](/develop/docs/dev-database-integrations-adbc), and work in a schema called `docs_sql_fn_date_time_day`. Set `QE_ENDPOINT`, `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY` and `VASTDB_BUCKET` ([SQL reference](/develop/docs/dev-database-reference-sql-index#example-connect-and-use-a-transaction)), and run the blocks in order in one Python session. Below each block is the output you should see.

This block connects, defines the `run()` and `plan()` helpers, and creates a table of five events. Each has a date (`d`), a timestamp (`ts`), a time of day (`t`) and a timestamp with nanoseconds (`tns`). Row 4 has no values, and row 5 is the last moment of 1969:

```
import os
import re

import adbc_driver_manager
import adbc_driver_manager.dbapi as dbapi
import adbc_driver_vastdb

BUCKET = os.environ["VASTDB_BUCKET"]
SCHEMA = f'"{BUCKET}/docs_sql_fn_date_time_day"'

conn = dbapi.connect(
    driver=adbc_driver_vastdb.get_driver_path(),
    db_kwargs={
        "vast.db.endpoint": os.environ["QE_ENDPOINT"],
        "vast.db.access_key": os.environ["AWS_ACCESS_KEY_ID"],
        "vast.db.secret_key": os.environ["AWS_SECRET_ACCESS_KEY"],
    },
    autocommit=True,
)

def server_message(e):
    """The server's message in a driver error, without the request ID or the echoed statement."""
    text = re.sub(r"\[req=0x[0-9a-f]+\]: ", "", str(e))
    found = (re.search(r'message: "(.*?)", request_id', text, re.S)
             or re.search(r"description: (.*)\)", text, re.S))
    message = found.group(1) if found else text.strip().splitlines()[-1]
    message = re.sub(r"^Plan query: '.*?' failed with: ", "", message, flags=re.S)
    message = re.split(r"\\n|\n", message)[0].replace('\\"', '"')
    return message.replace(BUCKET, "<bucket>")

def run(sql):
    """Run one statement. Print the rows it returns, or the error."""
    try:
        with conn.cursor() as cur:
            cur.execute(sql)
            rows = cur.fetchall() if cur.description else []
    except (adbc_driver_manager.Error, OSError) as e:
        print(f"{type(e).__name__}: {server_message(e)}")
    else:
        print(rows)

def plan(sql):
    """Print the plan without its ~N Rows lines: the table's row count, which can still be 0 just after an insert."""
    with conn.cursor() as cur:
        cur.execute("EXPLAIN " + sql)
        steps = cur.fetchall()[0][1]
    print("\n".join(line for line in steps.splitlines() if "Rows" not in line))

TABLE = f"{SCHEMA}.events"
run(f"CREATE SCHEMA IF NOT EXISTS {SCHEMA}")
run(f"CREATE TABLE IF NOT EXISTS {TABLE} (id BIGINT, d DATE, ts TIMESTAMP, t TIME, tns TIMESTAMP_NS)")
run(f"INSERT INTO {TABLE} VALUES "
    "(1, DATE '2026-02-28', TIMESTAMP '2026-02-28 23:15:00', TIME '23:15:00', "
    "CAST('2026-02-28 23:15:00.123456789' AS TIMESTAMP_NS)), "
    "(2, DATE '2026-03-01', TIMESTAMP '2026-03-01 08:30:45.123456', TIME '08:30:45.123456', "
    "CAST('2026-03-01 08:30:45.123456' AS TIMESTAMP_NS)), "
    "(3, DATE '2024-12-30', TIMESTAMP '2024-12-30 12:00:00', TIME '12:00:00', "
    "CAST('2024-12-30 12:00:00' AS TIMESTAMP_NS)), "
    "(4, NULL, NULL, NULL, NULL), "
    "(5, DATE '1969-12-31', TIMESTAMP '1969-12-31 23:59:59.999999', TIME '23:59:59.999999', "
    "CAST('1969-12-31 23:59:59.999999999' AS TIMESTAMP_NS))")
```

```
[]
[]
[(5,)]
```

## What it does

`day` returns the day of the month of a date or timestamp: `day(DATE &apos;2024-02-29&apos;)` is `29`. It works on every timestamp precision (`TIMESTAMP`, `TIMESTAMP_NS`, `TIMESTAMP_MS`, `TIMESTAMP_S`). `dayofmonth` returns the same values.

On an `INTERVAL`, `day` returns its days field as written. It doesn't turn hours into days or days into months:

| Interval | `day` | Why |
| --- | --- | --- |
| `INTERVAL &apos;1 year 2 months 3 days 04:05:06.789&apos;` | `3` | The days field |
| `INTERVAL &apos;40 days&apos;` | `40` | Not normalised into months |
| `INTERVAL &apos;36 hours&apos;` | `0` | 36 hours stay in the time part |
| `INTERVAL &apos;1 month&apos;` | `0` | A month isn't counted as days |

To split an interval into months, days and time the usual way first, use [normalized_interval](/develop/docs/dev-database-reference-sql-functions-date-time-normalized-interval): `normalized_interval(INTERVAL &apos;40 days 30 hours&apos;)` is 1 month, 11 days and 6 hours, so its `day` is `11`.

## NULLs and edge cases

| Input | Result |
| --- | --- |
| A NULL value (row 4) | `NULL` |
| `DATE &apos;infinity&apos;`, `TIMESTAMP &apos;-infinity&apos;` | `NULL` |
| `DATE &apos;0001-01-01&apos;` | `1` |
| `DATE &apos;9999-12-31&apos;` | `31` |
| 44 BC: `CAST(&apos;0044-03-15 (BC)&apos; AS DATE)` | `15` |
| `INTERVAL &apos;-1 year -2 months -3 days -04:05:06.789&apos;` | `-3` |
| `INTERVAL &apos;25 months 40 days 30 hours&apos;` | `40` |
| An untyped `NULL`: `day(NULL)` | Error: `Could not choose a best candidate function for the function call &quot;day(&quot;NULL&quot;)&quot;` |
| A `TIME` | Error: `No function matches the given name and argument types &apos;day(TIME)&apos;` |
| A string | Error: `Could not choose a best candidate function`; cast it to `DATE` first |
| A number, boolean, `BLOB` or list | Error: `No function matches the given name and argument types`, for example `&apos;day(INTEGER_LITERAL)&apos;` |

The same cases, as the cluster returns them. They use the table and helpers from [Set up the examples](/develop/docs/dev-database-reference-sql-functions-date-time-day#set-up-the-examples):

```
run(f"SELECT id, day(ts), dayofmonth(ts) FROM {TABLE} WHERE id = 4")
run("SELECT day(DATE 'infinity'), day(TIMESTAMP '-infinity'), day(DATE '0001-01-01'), day(DATE '9999-12-31'), "
    "day(CAST('0044-03-15 (BC)' AS DATE))")
run("SELECT day(INTERVAL '1 year 2 months 3 days 04:05:06.789'), day(INTERVAL '40 days'), day(INTERVAL '36 hours'), "
    "day(INTERVAL '1 month'), day(INTERVAL '-1 year -2 months -3 days -04:05:06.789'), "
    "day(INTERVAL '25 months 40 days 30 hours')")
run("SELECT normalized_interval(INTERVAL '40 days 30 hours'), day(normalized_interval(INTERVAL '40 days 30 hours'))")
run("SELECT day(NULL)")
run("SELECT day(TIME '13:45:30')")
run("SELECT dayofmonth('2024-02-29')")
run("SELECT day(123)")
```

```
[(4, None, None)]
[(None, None, 1, 31, 15)]
[(3, 40, 0, 0, -3, 40)]
[(MonthDayNano(months=1, days=11, nanoseconds=21600000000000), 11)]
InternalError: Binder Error: Could not choose a best candidate function for the function call "day("NULL")". In order to select one, please add explicit type casts.
InternalError: Binder Error: No function matches the given name and argument types 'day(TIME)'. You might need to add explicit type casts.
InternalError: Binder Error: Could not choose a best candidate function for the function call "dayofmonth(STRING_LITERAL)". In order to select one, please add explicit type casts.
InternalError: Binder Error: No function matches the given name and argument types 'day(INTEGER_LITERAL)'. You might need to add explicit type casts.
```

## Examples

The day of the month of each event, with `day` and `dayofmonth`:

```
run(f"SELECT id, day(d), day(ts), dayofmonth(ts) FROM {TABLE} ORDER BY id LIMIT 10")
```

```
[(1, 28, 28, 28), (2, 1, 1, 1), (3, 30, 30, 30), (4, None, None, None), (5, 31, 31, 31)]
```

Keep only the events on the first day of a month:

```
run(f"SELECT id FROM {TABLE} WHERE day(ts) = 1 ORDER BY id LIMIT 10")
```

```
[(2,)]
```

## How it runs in VAST

The Query Engine runs on the cluster's CNodes, and a query on a table starts with a scan in VAST DataBase (`VAST_SCAN`). VAST DataBase can evaluate a comparison of a column with a constant, but not a function of a column, so `day()` is always computed by the Query Engine:

- **In the `SELECT` list,** the scan reads the column and the Query Engine computes `day()` for each row it returns.
- **In `WHERE`,** the scan sends the column for every row, and a `FILTER` step in the Query Engine computes `day()` and keeps the rows that match. No rows are skipped in storage.

The plan of the `WHERE` example shows the `FILTER` step above the scan:

```
plan(f"SELECT id FROM {TABLE} WHERE day(ts) = 1")
```

```
┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             #1            │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│           FILTER          │
│    ────────────────────   │
│      ("day"(ts) = 1)      │
│                           │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│                           │
│        Projections:       │
│             ts            │
│             id            │
│                           │
└───────────────────────────┘
```

## Make it faster

- **For one particular day, filter on a range of the raw column.** `ts &gt;= TIMESTAMP &apos;2026-03-01 00:00:00&apos; AND ts &lt; TIMESTAMP &apos;2026-03-02 00:00:00&apos;` is 1 March 2026, and it's pushed down to VAST DataBase, so rows from other days never leave storage. On a `DATE` column, compare it with the date: `d = DATE &apos;2026-03-01&apos;`. The plan for the range has no `FILTER` step:

```
plan(f"SELECT id FROM {TABLE} WHERE ts >= TIMESTAMP '2026-03-01 00:00:00' AND ts < TIMESTAMP '2026-03-02 00:00:00'")
```

```
┌───────────────────────────┐
│         VAST_SCAN         │
│    ────────────────────   │
│    Function: VAST_SCAN    │
│      Projections: id      │
│                           │
└───────────────────────────┘
```

- **Store the day if you filter on it across months.** `day(ts) = 1` for every month has no single range. Compute it when you write the rows and keep it in its own column: `event_day = 1` is pushed down, `day(ts) = 1` isn't.

## Limitations and errors

- **No time-zone support.** On a `TIMESTAMP WITH TIME ZONE` value, such as `now()`, `day` fails with an `Extension Autoloading Error`. Cast the value to `TIMESTAMP` first; see [time zones](/develop/docs/dev-database-reference-sql-functions-date-time-time-zones).
- **Not for `TIME`.** `day(TIME ...)` fails with `No function matches the given name and argument types &apos;day(TIME)&apos;`.
- **`TIMESTAMP_NS` just before 1970.** Row 5's `tns`, `1969-12-31 23:59:59.999999999`, gives `1`, not `31`. Store such values as `TIMESTAMP` if you need their day.

```
run(f"SELECT id, day(tns), day(ts) FROM {TABLE} ORDER BY id LIMIT 10")
```

```
[(1, 28, 28), (2, 1, 1), (3, 30, 30), (4, None, None), (5, 1, 31)]
```

## Similar functions

| Function | Returns | Result for `DATE &apos;2024-02-29&apos;`, a Thursday (`day` gives `29`) |
| --- | --- | --- |
| [dayofweek](/develop/docs/dev-database-reference-sql-functions-date-time-dayofweek) | The day of the week, `0` (Sunday) to `6` | `4` |
| [isodow](/develop/docs/dev-database-reference-sql-functions-date-time-isodow) | The ISO day of the week, `1` (Monday) to `7` (Sunday) | `4` |
| [dayofyear](/develop/docs/dev-database-reference-sql-functions-date-time-dayofyear) | The day of the year, `1` to `366` | `60` |
| [dayname](/develop/docs/dev-database-reference-sql-functions-date-time-dayname) | The day's English name | `&apos;Thursday&apos;` |
| [last_day](/develop/docs/dev-database-reference-sql-functions-date-time-last-day) | The last day of the month | `2024-02-29` |

```
run("SELECT day(DATE '2024-02-29'), dayofweek(DATE '2024-02-29'), isodow(DATE '2024-02-29'), "
    "dayofyear(DATE '2024-02-29'), dayname(DATE '2024-02-29'), last_day(DATE '2024-02-29')")
```

```
[(29, 4, 4, 60, 'Thursday', datetime.date(2024, 2, 29))]
```

Clean up:

```
run(f"DROP TABLE IF EXISTS {TABLE}")
run(f"DROP SCHEMA IF EXISTS {SCHEMA}")
conn.close()
```

```
[]
[]
```

## See also

- [date_part](/develop/docs/dev-database-reference-sql-functions-date-time-date-part): any part of a value, by name.
- [Date and time functions](/develop/docs/dev-database-reference-sql-functions-date-time): all the date and time functions.
- [Where filters run](/develop/docs/dev-database-reference-sql-select#where-filters-run) and [pushdowns](/develop/docs/dev-database-reference-pushdowns).
- [EXPLAIN](/develop/docs/dev-database-reference-sql-explain): how to read a plan.