htpdate

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a tool to synchronize system time with web servers, for linux, windows and macos.

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# htpdate

**htpdate** is a tool to synchronize system time from web servers, supporting Windows, Linux, BSD, and macOS. It provides time calibration with 0.5-second accuracy and can be used as a backup measure or in cases where NTP (Network Time Protocol) is not available.

## Features

- **High Accuracy**: Provides time calibration with 0.5-second accuracy.
- **Cross-Platform**: Supports Windows, Linux, BSD, and macOS.
- **Flexible**: Can be used as a backup measure or in environments where NTP is not available.
- **Easy to Use**: Simple command-line interface with various options for customization.

## Why htpdate?

- **Ubiquity of Web Servers**: Websites are everywhere, making them a more accessible source for time synchronization compared to NTP servers.
- **Backup Measure**: htpdate can serve as a backup when NTP is not available.
- **Versatility**: Useful in scenarios where devices cannot use NTP.

## Installation

### Prerequisites

- **gcc**: Ensure you have the GNU Compiler Collection (gcc) installed.
- **libcurl-dev**: Ensure you have the libcurl development package installed.

### Build

To build htpdate, use the following command:

```sh
gcc -o htpdate htpdate.c -lcurl
```

## Usage

```sh
htpdate [options...] URLs...
```

### Options

- `-c, --count`: The number of requests for each URL. Default: 5.
- `-i, --interval`: The minimum milliseconds between requests. Default: 500.
- `-T, --timeout`: Total timeout value, milliseconds. Default: 100000.
- `-R, --transfer-timeout`: Transfer timeout value, milliseconds. Default: 60000.
- `-D, --dns-timeout`: The timeout value of the domain name resolution, milliseconds. Default: 30000.
- `-u, --user-agent`: Browser user agent name. Default: 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36'.
- `-m, --method`: HTTP method. Default: 'HEAD'.
- `-r, --retry`: Number of retries. Default: 0.
- `-k, --insecure`: Allow insecure server connections when using HTTPS. Default: false.
- `-a, --adjust`: Adjust system time if necessary. Default: false.
- `-t, --threshold`: At least how many milliseconds are considered to adjust system time. Default: 1500.
- `-h, --help`: Display this help text.
- `-V, --version`: Display the version of htpdate and exit.

## Time Synchronization Principle

According to the definition of RFC 7230/2822/2616, websites include a `Date` field in the response header, like this:

```
HTTP/2 200
Content-Type: text/html; charset=utf-8
Date: Wed, 03 Nov 2021 11:46:19 GMT
...
```

This `Date` field represents the moment the website processed the request, which is between the time we sent the request and the time we received the response. By assuming that the time to send the request and receive the response is equal, we can calculate the difference between local time and website time:

1. **Duration Calculation**:
   ```
   duration = received_at - sent_at
   ```

2. **Time Difference Calculation**:
   ```
   delta = server_time - received_at - duration / 2
   ```

3. **Precision Compensation**:
   Since the precision of the `Date` field can vary (e.g., `:23 GMT` could be `23.000` seconds or `23.999` seconds), we add a compensation of +0.5 seconds.

## Platform-Specific Handling

### Windows

On Windows, htpdate uses the `reg` command to query the short date format from the registry and constructs the appropriate `date` and `time` adjustment commands.

### Linux/MacOS

On Linux and macOS, htpdate uses the `date` command to adjust the system time.

## License

This project is licensed under the AGPL-3.0 License. See the [LICENSE](LICENSE) file for details.

---

**htpdate** is a powerful tool for time synchronization, offering a reliable alternative to NTP. Try it out and ensure your system time is always accurate!

htpdate

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