Why does my message send earlier than scheduled and Sent events appear before the scheduled time?
If your message is scheduled in fixed timezone or in UTC time, we are not sending the campaign earlier.
What you see is a side effect of the fast counters we use to track the data in almost realtime.
In order for us to deliver the message in time, we start preparing the message and building a queue of all users who should get it approximately 20-30 minutes before the actual sending.
Since that data is also used by the fast counters, you may see Sent numbers increasing shortly before the scheduled start time. This behavior can be observed in the Message summary or Campaign results page.
At the exact scheduling time, we call the service responsible for sending the message.