{
  "id": "days-until-the-31st",
  "version": "98c7e87c29db",
  "status": "published",
  "name": "Days Until the 31st",
  "question": "Days until the 31st",
  "summary": "Counts the days from today, or any date you choose, to the next 31st of a month, with its date and the 31st after it.",
  "category": "days",
  "subcategory": "countdowns",
  "url": "https://www.acalculator.org/days/days-until-the-31st",
  "markdown": "https://www.acalculator.org/days/days-until-the-31st.md",
  "kind": "function",
  "method": "Find the first date on or after the start date that is the 31st of its month, skipping months that have no 31st, then count the days from the start date to it.",
  "assumptions": [
    "On the 31st, the count is 0, and the 31st after is the 31st of the next month that has one.",
    "February, April, June, September, and November have no 31st, so the count skips to the 31st of the next month that has one. A leap year is a year divisible by 4, except a century year not divisible by 400 (2000 and 2400 are leap years; 1900 and 2100 are not).",
    "The start date itself is not counted: from the day before, the count is 1.",
    "Dates are calendar days with no time of day, so a time zone or a daylight-saving change never adds or loses a day.",
    "Start dates from January 1, 1900 to December 31, 2400 work."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "from": {
        "title": "Count from",
        "description": "The date to count from. Today, unless you change it.",
        "type": "string",
        "format": "date"
      }
    }
  },
  "outputs": {
    "days": {
      "label": "Days until the 31st",
      "description": "Whole days from the start date to the next 31st of a month; 0 when the start date is the 31st.",
      "format": "integer"
    },
    "date": {
      "label": "Next 31st",
      "description": "The first date on or after the start date that is the 31st of its month.",
      "format": "date-long"
    },
    "after": {
      "label": "The 31st after",
      "description": "The next 31st after that date.",
      "format": "date-long"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "from": "2026-10-19"
    },
    "outputs": {
      "days": 12,
      "date": "2026-10-31",
      "after": "2026-12-31"
    },
    "text": "From October 19, 2026, there are 12 days until the 31st, Saturday, October 31, 2026."
  },
  "examples": [
    {
      "given": {
        "from": "2026-10-19"
      },
      "expect": {
        "date": "2026-10-31",
        "days": 12,
        "after": "2026-12-31"
      },
      "source": "IETF RFC 3339, section 5.7 (days in each month) and Appendix C (the leap year rule) (https://www.rfc-editor.org/rfc/rfc3339); Python 3 datetime, walking one day at a time from the start date (docs/progress/WP-141/truth_days.py)"
    },
    {
      "given": {
        "from": "2026-12-31"
      },
      "expect": {
        "date": "2026-12-31",
        "days": 0,
        "after": "2027-01-31"
      },
      "source": "IETF RFC 3339, section 5.7 (days in each month) and Appendix C (the leap year rule) (https://www.rfc-editor.org/rfc/rfc3339); Python 3 datetime, walking one day at a time from the start date (docs/progress/WP-141/truth_days.py)"
    },
    {
      "given": {
        "from": "2027-02-01"
      },
      "expect": {
        "date": "2027-03-31",
        "days": 58,
        "after": "2027-05-31"
      },
      "source": "IETF RFC 3339, section 5.7 (days in each month) and Appendix C (the leap year rule) (https://www.rfc-editor.org/rfc/rfc3339); Python 3 datetime, walking one day at a time from the start date (docs/progress/WP-141/truth_days.py)"
    },
    {
      "given": {
        "from": "2028-02-15"
      },
      "expect": {
        "date": "2028-03-31",
        "days": 45,
        "after": "2028-05-31"
      },
      "source": "IETF RFC 3339, section 5.7 (days in each month) and Appendix C (the leap year rule) (https://www.rfc-editor.org/rfc/rfc3339); Python 3 datetime, walking one day at a time from the start date (docs/progress/WP-141/truth_days.py)"
    }
  ],
  "sources": [
    "IETF RFC 3339, Date and Time on the Internet: Timestamps, section 5.7 (the days in each month) and Appendix C (the leap year rule). https://www.rfc-editor.org/rfc/rfc3339",
    "U.S. Naval Observatory, Leap Years: the Gregorian leap year rule. https://aa.usno.navy.mil/faq/leap_years"
  ],
  "related": [
    "days-until-the-30th",
    "days-until-the-1st",
    "days-until",
    "days-from-today"
  ],
  "changelog": []
}