I am trying to use the ToDateTime function to use the integer Epoch value(Seconds elapsed from 1970 January 1). In the inbound Chat flow. But whenever I include ToDateTime(3600) in the update data the publish is failing.
ToDateTime(3600) is indeed a valid expression. However, I wasn't able to reproduce a publish failure in an inbound chat flow using this expression, so you may have something else going on in your flow that's preventing a publish. You'll probably want to open a support ticket for this issue.