# Days until the 21st

Counts the days from today, or any date you choose, to the next 21st of a month, with its date and the 21st after it.

- Page: https://www.acalculator.org/days/days-until-the-21st
- JSON spec: https://www.acalculator.org/days/days-until-the-21st.json
- Version: 970963ca03be

## Default answer

Example with the default inputs (Count from October 19, 2026) on the example date Monday, October 19, 2026: From October 19, 2026, there are 2 days until the 21st, Wednesday, October 21, 2026.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| from | Count from | The date to count from. Today, unless you change it. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| days | Days until the 21st | Whole days from the start date to the next 21st of a month; 0 when the start date is the 21st. |
| date | Next 21st | The first date on or after the start date that is the 21st of its month. |
| after | The 21st after | The next 21st after that date. |

## Method

Find the first date on or after the start date that is the 21st of its month, then count the days from the start date to it.

## Assumptions

- On the 21st, the count is 0, and the 21st after is the 21st of the next month that has one.
- Every month has a 21st, so the next one is never more than a month away.
- 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.

## Worked examples

1. from = 2026-10-19 gives date = 2026-10-21, days = 2, after = 2026-11-21. 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).
2. from = 2026-12-31 gives date = 2027-01-21, days = 21, after = 2027-02-21. 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).
3. from = 2027-02-01 gives date = 2027-02-21, days = 20, after = 2027-03-21. 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).
4. from = 2028-02-15 gives date = 2028-02-21, days = 6, after = 2028-03-21. 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).
5. from = 2027-01-21 gives date = 2027-01-21, days = 0, after = 2027-02-21. 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).

## FAQ

### How many days until the 21st?

It depends on the date, so the count above starts from today as your device shows it. From the 1st of a month that has a 21st, it is 20 days away; on the 21st itself the count is 0. Change “Count from” to count from another date.

### What if today is the 21st?

Then the count is 0, because the next 21st on or after today is today itself. The page also shows the 21st after it, which is 28 to 31 days later.

### Which months have a 21st?

All 12. Every month has at least 28 days, so every month has a 21st.

### How many times does the 21st come in a year?

12, one in every month.

### What day of the week will the 21st be?

It changes from month to month. A 31-day month moves the 21st 3 days later in the week, a 30-day month 2, a 29-day February 1, and a 28-day February 0. The page shows the weekday of the next 21st in its date.

### How are the days until the 21st counted?

From the start date (today, unless you change it) to the next 21st, without counting the start date, in calendar days. Weekends and holidays count like any other day.

## 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