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:

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

Response:

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

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

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

HTTP (no key)

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.

CLI

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

Get the client at /clients/cli/.

Related 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: Add a signed month count to a YYYY-MM-DD date, clamping the day to the target month.
• 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: Compute the calendar-aware elapsed years, months, and days between a birth date and an as-of date.
• ISO date business days count: Count the business days in an inclusive date range, skipping caller-supplied weekend weekdays and holidays.
• ISO date clamp: Clamp a YYYY-MM-DD calendar date into an inclusive [min, max] range.