ISO date month start

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

Return the first calendar day of the year-month of a YYYY-MM-DD date.

Use when you need to: iso date month start · first day of month · start of calendar month.

Supported

• iso date month start
• first day of month
• start of calendar month

Not supported

• ISO week start
• fiscal year start
• timezone local midnight
• datetime truncate
• last day of month
• add months

Behavior

• date is a proleptic Gregorian UTC calendar date YYYY-MM-DD with years 0001-9999.
• Output is the same year and month with day 01.
• No time zone, locale, or week-start adjustment is applied.

Input

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

Output

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

Limits

• min year: 1
• max year: 9999

Example

Request input:

{
  "date": "2026-03-15"
}

Response:

{
  "result": {
    "date": "2026-03-01"
  }
}

How to call it

MCP

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

{
  "id": "iso-date-month-start",
  "version": "1.0.0",
  "input": {
    "date": "2026-03-15"
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/iso-date-month-start/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"date":"2026-03-15"}'

The machine-readable contract is at /v1/tools/iso-date-month-start/versions/1.0.0.

CLI

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

Get the client at /clients/cli/.

Related tools

• ISO date month end: Return the last valid calendar day of the year-month of a YYYY-MM-DD date.
• ISO date quarter bounds: Return the calendar quarter number and the first/last dates of the quarter containing a date.
• ISO date day of year: Return the ordinal day number (1-366) of a YYYY-MM-DD date within its calendar year.
• ISO date days in month: Return the number of calendar days in a given proleptic Gregorian year and month.
• ISO date from day of year: Convert a calendar year and ordinal day number (1-366) to a YYYY-MM-DD date.
• ISO date last weekday of month: Return the date of the last occurrence of an ISO weekday within a calendar month.