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Dart HttpDate

last modified April 4, 2025

The HttpDate class in Dart provides utilities for parsing and formatting dates in HTTP headers. It handles RFC 1123, RFC 850, and ANSI C date formats.

HttpDate is essential for working with HTTP headers like Last-Modified, Expires, and Date. It's part of Dart's dart:core library.

Basic Definition

HttpDate is a utility class for HTTP date handling. It contains static methods for parsing and formatting dates according to HTTP standards.

Key features include parsing multiple date formats, strict RFC compliance, and thread-safe operations. It's used internally by Dart's HTTP packages.

Parsing HTTP Dates

This example shows how to parse HTTP date strings into DateTime objects.

main.dart 
import 'dart:core';

void main() {
  var rfc1123 = 'Sun, 06 Nov 1994 08:49:37 GMT';
  var rfc850 = 'Sunday, 06-Nov-94 08:49:37 GMT';
  var asctime = 'Sun Nov  6 08:49:37 1994';
  
  var date1 = HttpDate.parse(rfc1123);
  var date2 = HttpDate.parse(rfc850);
  var date3 = HttpDate.parse(asctime);
  
  print(date1); // 1994-11-06 08:49:37.000Z
  print(date2); // 1994-11-06 08:49:37.000Z
  print(date3); // 1994-11-06 08:49:37.000Z
}

We parse three different HTTP date formats into DateTime objects. HttpDate handles all standard formats automatically, returning UTC DateTime objects. 

$ dart main.dart
1994-11-06 08:49:37.000Z
1994-11-06 08:49:37.000Z
1994-11-06 08:49:37.000Z

Formatting HTTP Dates

This example demonstrates formatting DateTime objects to HTTP date strings.

main.dart 
import 'dart:core';

void main() {
  var date = DateTime.utc(1994, 11, 6, 8, 49, 37);
  
  var httpDate = HttpDate.format(date);
  print(httpDate); // Sun, 06 Nov 1994 08:49:37 GMT
  
  var now = DateTime.now().toUtc();
  print(HttpDate.format(now)); // Current time in RFC 1123 format
}

We format DateTime objects to RFC 1123 compliant strings. HttpDate.format always produces dates in this standard format used by HTTP headers. 

$ dart main.dart
Sun, 06 Nov 1994 08:49:37 GMT
Mon, 04 Apr 2025 14:30:00 GMT

Handling HTTP Headers

This example shows practical use of HttpDate with HTTP headers.

main.dart 
import 'dart:core';

void main() {
  // Simulate HTTP response headers
  var headers = {
    'date': 'Mon, 04 Apr 2025 12:00:00 GMT',
    'last-modified': 'Fri, 01 Jan 2021 00:00:00 GMT'
  };
  
  var date = HttpDate.parse(headers['date']!);
  var lastModified = HttpDate.parse(headers['last-modified']!);
  
  print('Response date: $date');
  print('Last modified: $lastModified');
  
  // Check if resource is fresh (less than 1 day old)
  var isFresh = date.difference(lastModified) < Duration(days: 1);
  print('Resource is fresh: $isFresh');
}

We parse standard HTTP date headers and perform date comparisons. This is typical for cache validation and conditional requests in HTTP clients. 

$ dart main.dart
Response date: 2025-04-04 12:00:00.000Z
Last modified: 2021-01-01 00:00:00.000Z
Resource is fresh: false

Error Handling

This example demonstrates proper error handling when parsing dates.

main.dart 
import 'dart:core';

void main() {
  var invalidDate = 'Monday, 04-Apr-25 12:00:00 GMT';
  
  try {
    var date = HttpDate.parse(invalidDate);
    print(date);
  } on FormatException catch (e) {
    print('Failed to parse date: ${e.message}');
    print('Input: ${e.source}');
  }
  
  // Fallback to current time if parsing fails
  var date = _parseHttpDateWithFallback(invalidDate);
  print('Using date: $date');
}

DateTime _parseHttpDateWithFallback(String dateStr) {
  try {
    return HttpDate.parse(dateStr);
  } on FormatException {
    return DateTime.now().toUtc();
  }
}

We handle FormatException when parsing malformed dates. The example shows a robust approach with fallback to default values when parsing fails. 

$ dart main.dart
Failed to parse date: Invalid HTTP date
Input: Monday, 04-Apr-25 12:00:00 GMT
Using date: 2025-04-04 14:30:00.123Z

Custom Date Validation

This example shows how to validate and normalize HTTP dates.

main.dart 
import 'dart:core';

void main() {
  var dates = [
    'Sun, 06 Nov 1994 08:49:37 GMT',
    'Sunday, 06-Nov-94 08:49:37 GMT',
    'Sun Nov  6 08:49:37 1994',
    '06 Nov 1994 08:49:37 GMT',
    'Invalid Date'
  ];
  
  for (var dateStr in dates) {
    try {
      var date = HttpDate.parse(dateStr);
      var normalized = HttpDate.format(date);
      print('Valid: $normalized');
    } on FormatException {
      print('Invalid: $dateStr');
    }
  }
}

We validate multiple date strings and normalize valid ones to RFC 1123 format. This demonstrates a complete validation and normalization pipeline for HTTP dates. 

$ dart main.dart
Valid: Sun, 06 Nov 1994 08:49:37 GMT
Valid: Sun, 06 Nov 1994 08:49:37 GMT
Valid: Sun, 06 Nov 1994 08:49:37 GMT
Invalid: 06 Nov 1994 08:49:37 GMT
Invalid: Invalid Date

Best Practices

Source

Dart HttpDate Documentation

This tutorial covered Dart's HttpDate class with practical examples showing date parsing, formatting, error handling, and HTTP header integration.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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