That really depends on how you are ordering the posts in the first place, for example on the event archive page.
Do you also have an end time field?
Are you just ordering the posts by the start time?
if you are just using the single field then WP is ordering them first by this field and then in the order that the posts were created. This would mean that the next post of the same time would be the post with a larger ID and the previous post would be a post with a smaller ID.
But I’d need more information about what you consider the previous and next post based on the order you’d expect them to appear in a list.
