# Add or Subtract Days Calculator

> Adds a signed number of days to an ISO date and returns the resulting date and its weekday, using Gregorian calendar arithmetic in UTC.

- Calculator id: `add-days` · Category: Everyday Life (`everyday`) · Tool name: `calculate_date_plus_days`
- Canonical page: https://tttkmbb.com/everyday/add-days · This document: https://tttkmbb.com/everyday/add-days.md · JSON definition: https://tttkmbb.com/everyday/add-days.json

## Purpose

Adds a signed number of days to an ISO date and returns the resulting date and its weekday, using Gregorian calendar arithmetic in UTC.

**Use when:** You need the date that is N days after (or before) a given date, e.g. a deadline 90 days from today or a return date 14 days after departure.

**Do not use when:** You need the number of days between two known dates (use date-difference) or want to skip weekends when counting (not supported here).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `start_date` | date |  | optional, default "today" | Date to start from (YYYY-MM-DD); defaults to the current UTC date. |
| `days` | integer | days | required | Number of days to add; negative values subtract. (min -1000000, max 1000000) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `result_date` | date |  | start_date + days. |
| `weekday` | string |  | Day of the week of the result date. |
| `offset_text` | string |  | The offset written as days and as weeks and days, before or after start_date. |

## Formula

`result_date = start_date + days (calendar days, no weekend skipping); weekday = day of week of result_date`

## Data Sources

- Gregorian calendar – leap-year rule and month lengths (Wikipedia) — https://en.wikipedia.org/wiki/Gregorian_calendar (reference, retrieved 2026-09-23)
- ISO 8601 – Date and time format (Wikipedia) — https://en.wikipedia.org/wiki/ISO_8601 (reference, retrieved 2026-09-23)

Data freshness: `daily` (max_age_seconds 86400). Depends on the current date unless the date is supplied.

## API

- `GET https://tttkmbb.com/api/v1/calculate/add-days?days=…`
- `POST https://tttkmbb.com/api/v1/calculate/add-days` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/add-days · OpenAPI operationId `calculate_date_plus_days` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "add-days", "inputs": {…}}`

## Example

- 2026-09-23 + 90 days: inputs `{"start_date":"2026-09-23","days":90}` → `{"result_date":"2026-12-22","weekday":"Tuesday","offset_text":"90 days (12 weeks 6 days) after 2026-09-23"}`
- 2026-03-01 − 30 days: inputs `{"start_date":"2026-03-01","days":-30}` → `{"result_date":"2026-01-30","weekday":"Friday","offset_text":"30 days (4 weeks 2 days) before 2026-03-01"}`

```
GET https://tttkmbb.com/api/v1/calculate/add-days?start_date=2026-09-23&days=90
```

## Limitations

You need the number of days between two known dates (use date-difference) or want to skip weekends when counting (not supported here). All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Are weekends or holidays skipped?**

No. Every calendar day counts. For business-day counting between two dates see date-difference.

**Is the start date day 0 or day 1?**

Day 0: adding 1 day to Monday gives Tuesday. For an inclusive convention (start date counts as day 1) add one day fewer.

## Related

- [Date Difference Calculator](https://tttkmbb.com/everyday/date-difference.md) — Inverse operation: days between two dates.
- [Days Until Calculator](https://tttkmbb.com/everyday/days-until.md) — Countdown from today to a target date.