happytails.ai

Week number calculator

Open Week number calculator on Happytails.ai. This view has the same page content and links, with explicit tool capabilities. Read the processing notes before using a tool.

Tool capabilities

{
  "id": "week-number-calculator",
  "category": "date-time-tools",
  "built": true,
  "requirements": "A date's calendar year can differ from its ISO week-year around New Year. This tool uses ISO 8601 week numbering, not Sunday-start calendars or organization-specific fiscal weeks.",
  "implementation": {
    "fields": [
      {
        "key": "date",
        "label": "Calendar date",
        "value": "2026-10-06",
        "type": "date"
      }
    ],
    "input": false,
    "live": true,
    "presentation": "metrics",
    "note": "ISO 8601: weeks start on Monday; week 1 contains January 4. Dates near New Year can belong to the preceding or following ISO week-year.",
    "experience": {
      "automatic": true,
      "single": false,
      "inputMaxLength": 1000000,
      "inputMode": "text",
      "label": "Your text",
      "placeholder": "Type or paste your input…",
      "help": "Change the input or settings to update your result. Results update as you type. Up to 1,000,000 characters.",
      "helpParts": [
        "Change the input or settings to update your result.",
        "Results update as you type.",
        "Up to 1,000,000 characters."
      ],
      "action": null
    },
    "mode": "automatic",
    "interactive": false
  },
  "execution": "Browser interface",
  "api": null,
  "browser_agent": {
    "supported": true,
    "name": "run_current_tool",
    "discovery": "WebMCP on the human page in a supporting browser",
    "verification": "See audit/API-AUDIT.md; availability is not verification"
  }
}

Page guide

Find a date's ISO week number and week-year.

How to use Week number calculator
  1. Enter the values you want to calculate and choose the relevant units or settings.
  2. The result updates as you type. Adjust the available settings if needed.
  3. Review the result, then use Copy result or a download link when available.

Useful next steps

Day of the week calculatorBusiness daysAdd days

How to use it

Choose a calendar date. ISO weeks start on Monday, and the first week of the year contains January 4. The result also shows the ISO weekday, Monday 1 through Sunday 7.

A worked example

January 1, 2021 belongs to ISO week 53 of 2020. January 4, 2021 starts ISO week 1 of 2021.

What to know

A date's calendar year can differ from its ISO week-year around New Year. This tool uses ISO 8601 week numbering, not Sunday-start calendars or organization-specific fiscal weeks.