Storage and output
All timestamps are stored and returned in UTC, in ISO 8601 format:updated_time, first_campaign_email_sent, last_campaign_email_sent, last_reply_received, and the LinkedIn activity timestamps (first_linkedin_message_sent, last_linkedin_connection_accepted, and so on) all follow this format. A field with no recorded value is returned as null.
Sending dates
When you send a date or datetime value, for example a custom field of type date, use ISO 8601:Sorting
List endpoints that expose an implicit order (for example contacts) sort byupdated_time descending, most recently updated first, with id as a tiebreaker.
