# Add days

- Human page: https://happytails.ai/en/date-time-tools/add-days/
- Markdown: https://happytails.ai/en/date-time-tools/add-days.md
- Structured JSON: https://happytails.ai/agents/en/date-time-tools/add-days/index.json
- Visibility: public

## Tool capabilities

| Property | Value |
| --- | --- |
| Tool ID | `add-days` |
| Category | date-time-tools |
| Implementation | Registered implementation. Registration alone is not a production-quality guarantee. |
| Browser execution | Automatic updates after valid input or file selection. |
| Browser processing | Browser; see tool notes for network behavior. |
| HTTP API | /api/v1/tools/add-days/run |
| Browser-agent interface | run_current_tool |

### Inputs and settings

No free-text input is required. Use the file inputs and/or settings below.

| Setting | Meaning | Control type | Default | Constraints |
| --- | --- | --- | --- | --- |
| `date` | Date | date | "2026-01-01" |  |
| `days` | Days to add (negative subtracts) | number | 30 |  |

### HTTP contract

API calls process supplied data on the server. The website origin is `https://happytails.ai`. Server API hosting is not connected on the public website yet. Use your local server origin to test these requests.

```http
POST https://happytails.ai/api/v1/tools/add-days/run
Content-Type: application/json
```

[Detailed operation reference](https://happytails.ai/en/api/tools/add-days.md) · [Shared API guide](https://happytails.ai/en/api.md)

#### Limits

| Limit | Value |
| --- | --- |
| request_bytes | 8388608 |
| input_characters | 1000000 |
| files_bytes | 5242880 |
| max_files | 0 |
| timeout_seconds | 12 |

#### Complete input schema

```json
{
  "type": "object",
  "additionalProperties": false,
  "properties": {
    "input": {
      "type": "string",
      "maxLength": 1000000,
      "default": "",
      "description": "Plain text input. File tools use files instead unless otherwise documented."
    },
    "options": {
      "type": "object",
      "additionalProperties": false,
      "properties": {
        "date": {
          "type": "string",
          "description": "Date",
          "default": "2026-01-01"
        },
        "days": {
          "type": "number",
          "description": "Days to add (negative subtracts)",
          "default": 30.0
        }
      }
    }
  }
}
```

#### Complete output schema

```json
{
  "type": "object",
  "required": [
    "tool",
    "result"
  ],
  "properties": {
    "tool": {
      "type": "string"
    },
    "result": {
      "type": "object",
      "required": [
        "text",
        "files"
      ],
      "properties": {
        "text": {
          "type": [
            "string",
            "null"
          ]
        },
        "data": {
          "description": "Parsed JSON when the textual result is JSON."
        },
        "name": {
          "type": "string"
        },
        "mime": {
          "type": "string"
        },
        "files": {
          "type": "array",
          "items": {
            "type": "object",
            "required": [
              "name",
              "mime",
              "bytes",
              "base64"
            ],
            "properties": {
              "name": {
                "type": "string"
              },
              "mime": {
                "type": "string"
              },
              "bytes": {
                "type": "integer"
              },
              "base64": {
                "type": "string",
                "contentEncoding": "base64"
              }
            }
          }
        }
      }
    }
  }
}
```

#### Error and retry handling

Failures return a non-200 status and `error.code` plus `error.message`. Correct invalid input before retrying; retry server-busy responses with bounded backoff. Returned files contain Base64 data, not persistent download URLs. Decode and inspect the output before treating conversion as successful. See the shared API guide for the complete status and timeout rules.

### Browser-agent workflow

1. Open the human page in a browser that supports WebMCP.
2. Discover the interface exposed by that page; use its actual schema.
3. Select files on the page first when the tool requires files.
4. Call `run_current_tool` with valid input and settings.
5. Inspect the returned result or error and the displayed output. Retrieve files from the displayed download links.

Calls do not automatically copy or download results. Browser permissions still apply. Interface availability alone is not proof of successful execution.

### Planning specification

Add/subtract calendar days and expose timezone/calendar assumptions.

This describes intended requirements. Use the implementation notes, interface schema and observed output to determine current support.

### Complete machine-readable capability record

```json
{
  "id": "add-days",
  "category": "date-time-tools",
  "built": true,
  "requirements": "Add/subtract calendar days and expose timezone/calendar assumptions.",
  "implementation": {
    "fields": [
      {
        "key": "date",
        "label": "Date",
        "value": "2026-01-01",
        "type": "date"
      },
      {
        "key": "days",
        "label": "Days to add (negative subtracts)",
        "value": 30,
        "type": "number"
      }
    ],
    "input": false,
    "experience": {
      "automatic": true,
      "single": false,
      "label": "Your text",
      "placeholder": "Hello world!",
      "help": "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": "POST /api/v1/tools/add-days/run",
  "api": {
    "id": "add-days",
    "name": "Add days",
    "category": "date-time-tools",
    "description": "Add/subtract calendar days and expose timezone/calendar assumptions.",
    "notes": "",
    "human_path": "/en/date-time-tools/add-days/",
    "method": "POST",
    "endpoint": "/api/v1/tools/add-days/run",
    "schema_url": "/api/v1/tools/add-days",
    "input_schema": {
      "type": "object",
      "additionalProperties": false,
      "properties": {
        "input": {
          "type": "string",
          "maxLength": 1000000,
          "default": "",
          "description": "Plain text input. File tools use files instead unless otherwise documented."
        },
        "options": {
          "type": "object",
          "additionalProperties": false,
          "properties": {
            "date": {
              "type": "string",
              "description": "Date",
              "default": "2026-01-01"
            },
            "days": {
              "type": "number",
              "description": "Days to add (negative subtracts)",
              "default": 30.0
            }
          }
        }
      }
    },
    "output_schema": {
      "type": "object",
      "required": [
        "tool",
        "result"
      ],
      "properties": {
        "tool": {
          "type": "string"
        },
        "result": {
          "type": "object",
          "required": [
            "text",
            "files"
          ],
          "properties": {
            "text": {
              "type": [
                "string",
                "null"
              ]
            },
            "data": {
              "description": "Parsed JSON when the textual result is JSON."
            },
            "name": {
              "type": "string"
            },
            "mime": {
              "type": "string"
            },
            "files": {
              "type": "array",
              "items": {
                "type": "object",
                "required": [
                  "name",
                  "mime",
                  "bytes",
                  "base64"
                ],
                "properties": {
                  "name": {
                    "type": "string"
                  },
                  "mime": {
                    "type": "string"
                  },
                  "bytes": {
                    "type": "integer"
                  },
                  "base64": {
                    "type": "string",
                    "contentEncoding": "base64"
                  }
                }
              }
            }
          }
        }
      }
    },
    "processing": "Server; same transformation code as browser tools",
    "limits": {
      "request_bytes": 8388608,
      "input_characters": 1000000,
      "files_bytes": 5242880,
      "max_files": 0,
      "timeout_seconds": 12
    }
  },
  "agent_processing": "Server; same transformation code as browser tools",
  "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

Add/subtract calendar days and expose timezone/calendar assumptions..

## Useful next steps

- [Date difference](https://happytails.ai/en/date-time-tools/date-difference/): Inclusive/exclusive counting, leap years, calendar dates versus elapsed time. ([Markdown](https://happytails.ai/en/date-time-tools/date-difference.md))

- [Business days](https://happytails.ai/en/date-time-tools/business-days/): Custom weekends first. ([Markdown](https://happytails.ai/en/date-time-tools/business-days.md))

- [Time zone converter](https://happytails.ai/en/date-time-tools/time-zone-converter/): IANA zones, date selection and DST ambiguity. ([Markdown](https://happytails.ai/en/date-time-tools/time-zone-converter.md))

## How to use Add days

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.

## Working with add days

Date calculations can depend on calendar rules, time zones and whether endpoints are included. A duration in hours is different from a number of calendar days when daylight-saving changes occur.

## Common questions

### Are all days exactly 24 hours long?

Elapsed time across a daylight-saving transition can differ from a simple calendar-day count. Use the measure relevant to your schedule.

### Why can the same timestamp show different dates?

A timestamp represents an instant, while a calendar date is interpreted in a time zone. Near midnight, the same instant can fall on different local dates.

Continue your workflow: [Date difference](https://happytails.ai/en/date-time-tools/date-difference/), [Business days](https://happytails.ai/en/date-time-tools/business-days/), [Time zone converter](https://happytails.ai/en/date-time-tools/time-zone-converter/).