Survey123 Epoch Time to Sharepoint List Microsoft Flow

[Smithhy]

I am trying to convert Epoch Time from a date/time attribute in Survey123 to show as a readible format in a Sharepoint List. The flow keeps failing and I keep getting error messages. 

My flow has three parts to it: 

1. When a Survey Reponse is Submitteted - 2. Convert time Zone - 3. Create Entry Into Sharepoint List

The flow will work when I use the Date/Time attribute Dynamic Content, however it displays in Epoch time. 

To try get a readible format, I have added the Convert Time Zone step. In the Base Time I have the following Expression: 

addseconds('1970-1-1',Div(triggerBody()?['feature']?['attributes']?['StartDateTime']),'yyyy-MM-dd')

This keeps failing and I can't figure out why. Can anyone help? 

Thanks.