Hi there,
When you are using POST to post the event, it is going to pick up time stamp defined in your Event model.
Check your models.py. Go to the section where Event is defined.
You will find start element to have auto_now_add=True. This is the reason that whever any event is added, it will be UTC time stamp for the event.
start = db.DateTimeProperty(required=True, auto_now_add=True)
BTW, why are you trying to add Event date manually with the specific timestamp. Objective of using API should be to push event to stashboard as and when they occur.
If this is an exceptional requirement and you need to do this once a while to correct your data, you can manually update start date from SDK console.
Regards,
Anurag