Scheduled time zone via API appears as UTC in Dashboard. How to display correct timezone?

I have entered the following date and time into the payload to schedule an SMS message in the Pacific time. The message is scheduled to be sent at 3pm Pacific time today 5-3-2024.

payload = {
"schedule_at": "2024-05-03T15:00:00-07:00",

Within the dashboard, it shows the scheduled SMS message via API as:

Friday, May 3 at 10:00 pm (America/New_York)

This time is UTC. When clicking "Edit scheduled message" within the scheduled tab, no timezone is selected for the "Edit Timezone" field. If I manually select the Pacific time, I am unable to select 3pm from the drop-down time values. I think it is because my system OS is set to the Eastern time zone.

Am I entering the payload date and time in the correct format? How do I set the timezone via API? In this case, Pacific Time.