# ISO date add

`iso-date-add` · version 1.0.0 · Dates & times (UTC, ISO 8601) · free, no key needed

Add a signed whole-day delta to a strict YYYY-MM-DD calendar date.

**Use when you need to: iso date add · add days to date · date plus days.**

## Supported

- iso date add
- add days to date
- date plus days

## Not supported

- timezones
- weeks
- months
- locale
- Date local time

## Behavior

- Date must match /^[0-9]{4}-[0-9]{2}-[0-9]{2}$/ and be a real proleptic Gregorian calendar day with year 0001..9999.
- Leap years: year % 4 == 0 && (year % 100 != 0 || year % 400 == 0). 0000 is not in range. 2000-02-29 valid, 1900-02-29 invalid, 2023-02-29 invalid.
- days is a canonical integer string (same integer rules) whose BigInt absolute value is <= 3652425.
- Do not use JavaScript Date for year 0-99 pitfalls. Use integer day counts (Rata Die or equivalent) in UTC civil calendar with no time-of-day.
- Result must remain in 0001-01-01 .. 9999-12-31 or reject.

## Input

- `date` (string, required): min length 10; max length 10
- `days` (string, required): min length 1; max length 257

## Output

- `date` (string, required): min length 10; max length 10

## Limits

- year min: 1
- year max: 9999
- max abs day delta: 3652425

## Example

Request input:

```json
{
  "date": "2026-09-10",
  "days": "1"
}
```

Response:

```json
{
  "result": {
    "date": "2026-09-11"
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "iso-date-add",
  "version": "1.0.0",
  "input": {
    "date": "2026-09-10",
    "days": "1"
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/iso-date-add/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"date":"2026-09-10","days":"1"}'
```

The machine-readable contract is at [/v1/tools/iso-date-add/versions/1.0.0](/v1/tools/iso-date-add/versions/1.0.0).

### CLI

```sh
node cli.mjs run iso-date-add 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

- [ISO date business day add](/tools/iso-date-business-day-add): Add a signed count of business days to a date, skipping caller-supplied weekend weekdays and holidays.
- [ISO date add months](/tools/iso-date-add-months): Add a signed month count to a YYYY-MM-DD date, clamping the day to the target month.
- [ISO date add years](/tools/iso-date-add-years): Add a signed whole-year delta to a YYYY-MM-DD date, clamping the day if 29 February has no match.
- [ISO date age](/tools/iso-date-age): Compute the calendar-aware elapsed years, months, and days between a birth date and an as-of date.
- [ISO date business days count](/tools/iso-date-business-days-count): Count the business days in an inclusive date range, skipping caller-supplied weekend weekdays and holidays.
- [ISO date clamp](/tools/iso-date-clamp): Clamp a YYYY-MM-DD calendar date into an inclusive [min, max] range.