ISO date add months

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

Add a signed month count to a YYYY-MM-DD date, clamping the day to the target month.

Use when you need to: iso date add months · add calendar months to an iso date · shift iso date by months.

Supported

• iso date add months
• add calendar months to an iso date
• shift iso date by months

Not supported

• add days
• add seconds
• add years as a separate unit
• time zones
• locale month names
• fiscal calendars
• datetime with time
• ISO week arithmetic

Behavior

• date is a proleptic Gregorian UTC calendar date YYYY-MM-DD with years 0001-9999.
• months is a canonical integer string with at most 6 digits excluding sign; -0 is accepted as 0.
• Month arithmetic uses index = year*12 + (month-1) + months, then year-prime = floor(index/12) and month-prime in 1..12.
• The day is clamped to the last valid day of the target month (31 January plus one month is 28 or 29 February).
• The result year must stay in 0001-9999.
• Output date is zero-padded YYYY-MM-DD.

Input

• date (string, required): max length 10; pattern ^\d{4}-\d{2}-\d{2}$
• months (string, required): max length 7; pattern ^-?(0|[1-9][0-9]*)$

Output

• date (string, required): max length 10; pattern ^\d{4}-\d{2}-\d{2}$

Limits

• min year: 1
• max year: 9999
• max months digits: 6

Example

Request input:

{
  "date": "2026-01-31",
  "months": "1"
}

Response:

{
  "result": {
    "date": "2026-02-28"
  }
}

How to call it

MCP

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

{
  "id": "iso-date-add-months",
  "version": "1.0.0",
  "input": {
    "date": "2026-01-31",
    "months": "1"
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/iso-date-add-months/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"date":"2026-01-31","months":"1"}'

The machine-readable contract is at /v1/tools/iso-date-add-months/versions/1.0.0.

CLI

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

Get the client at /clients/cli/.

Related 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-time add seconds: Add a signed whole-second delta to a strict UTC ISO-8601 datetime.
• ISO date add: Add a signed whole-day delta to a strict YYYY-MM-DD calendar date.
• ISO date age: Compute the calendar-aware elapsed years, months, and days between a birth date and an as-of date.
• ISO date business day add: Add a signed count of business days to a date, skipping caller-supplied weekend weekdays and holidays.
• ISO date range split by month: Split an inclusive [start, end] date range into per-calendar-month sub-ranges.