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.

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年11月06日 08:49:37.000Z
 print(date2); // 1994年11月06日 08:49:37.000Z
 print(date3); // 1994年11月06日 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年11月06日 08:49:37.000Z
1994年11月06日 08:49:37.000Z
1994年11月06日 08:49:37.000Z

Formatting HTTP Dates

This example demonstrates formatting DateTime objects to HTTP date strings.

import 'dart:core';
void main() {
 var date = DateTime.utc(1994, 11, 6, 8, 49, 37);
 
 var httpDate = HttpDate.format(date);
 print(httpDate); // 1994年11月06日 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
1994年11月06日 08:49:37 GMT
2025年4月04日 14:30:00 GMT

Handling HTTP Headers

This example shows practical use of HttpDate with HTTP headers.

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年04月04日 12:00:00.000Z
Last modified: 2021年01月01日 00:00:00.000Z
Resource is fresh: false

Error Handling

This example demonstrates proper error handling when parsing dates.

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年04月04日 14:30:00.123Z

Custom Date Validation

This example shows how to validate and normalize HTTP dates.

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: 1994年11月06日 08:49:37 GMT
Valid: 1994年11月06日 08:49:37 GMT
Valid: 1994年11月06日 08:49:37 GMT
Invalid: 06 Nov 1994 08:49:37 GMT
Invalid: Invalid Date

Best Practices

• Always UTC: HttpDate works with UTC dates only
• Error handling: Always catch FormatException when parsing
• Normalization: Format to RFC 1123 for consistency
• Performance: Reuse parsed DateTime objects when possible
• Validation: Validate dates from untrusted sources

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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