RFC 2822 Date Format

RFC 2822 defines the date and time format originally specified for email message headers. Its format is human-readable and includes the day of the week: ddd, DD MMM YYYY HH:mm:ss ±HHMM. You encounter it in email Date: headers, some HTTP headers, and RSS feeds.

Format breakdown:

Mon, 15 Jan 2024 09:30:00 +0000
│     │  │    │    │        │
│     │  │    │    │        └── Timezone offset (no colon)
│     │  │    │    └── Time (24-hour)
│     │  │    └── Year (4-digit)
│     │  └── Month (3-letter abbreviation)
│     └── Day of month
└── Day of week (3-letter abbreviation)

Key differences from ISO 8601:

Feature	ISO 8601	RFC 2822
Month format	Numeric (01-12)	Abbreviated name (Jan-Dec)
Day of week	Not included	Included (optional)
Separator	T between date/time	Space between date/time
Timezone	Z or ±HH:MM	±HHMM (no colon)
Sortable	Yes	No (month names break sorting)

Generating RFC 2822 in code:

// JavaScript
new Date().toUTCString()
// "Mon, 15 Jan 2024 09:30:00 GMT"

# Python
from email.utils import formatdate
formatdate(timeval=None, localtime=False, usegmt=True)

Common pitfalls:

Legacy systems sometimes use timezone abbreviations like "EST" or "PST" instead of numeric offsets. These abbreviations are ambiguous (CST can mean Central Standard Time in the US, China Standard Time, or Cuba Standard Time) and should be avoided. Always use the numeric offset format +0000 for unambiguous parsing.

When to use which: Prefer ISO 8601 for APIs and data storage. Use RFC 2822 when required by a protocol specification (SMTP, certain HTTP headers, RSS). If you have a choice, ISO 8601 is the better default because it sorts naturally and is less ambiguous.