UID

UID provides a globally unique identifier for every iCalendar component.

Format

Recommended: UUID + domain.

UID:550e8400-e29b-41d4-a716-446655440000@example.com

Requirements

• Globally unique across all systems and users.
• Same UID = same logical event (calendar apps update, don’t duplicate).
• Must never change during the component’s lifecycle.

Best Practices

• Use UUID v4 for strong uniqueness.
• Append a domain (uuid@domain).
• Never reuse UIDs.
• UIDs are case-sensitive.

RECURRENCE-ID

Modified recurring instances use the same UID with RECURRENCE-ID:

BEGIN:VEVENT
UID:meeting@example.com
RECURRENCE-ID:20240122T100000Z
DTSTART:20240122T140000Z
SUMMARY:Meeting (moved)
END:VEVENT