Specifies the date and time to send in UTC (UTC+0). Use null to send immediately. If a past date/time is provided, it defaults to the current time (NOW). Accepted formats are YYYY-MM-DD, YYYY-MM-DD HH:mm, YYYY-MM-DD HH:mm:ss, YYYY-MM-DD HH:mm:ss Z, YYYY-MM-DD HH:mm:ssZ, or the ISO-8601 format YYYY-MM-DDTHH:mm:ssZ (where Z indicates UTC and timezone offsets such as +00:00 are also supported). Invalid formats or values will be set to null.