Hey @Carmen McGrady Racheal from the support engineer team here 

I'm afraid at the moment, there isn't anyway to backdate your News items, so they will all be dated for the day you posted them. However, i will pass the feedback to our product as feature request!

This would be really useful for me also

@Racheal Hey! Has any further consideration been given to this? This would really drive adoption of the news feed on our team – at the moment, it is hard to consider expanding our use of it without this. 

@Alex Panagis There haven’t been any recent changes to this behavior. I don’t know of any current plans to add this functionality, but that doesn’t mean it won’t be available down the road. I just don’t see it on our roadmap at the moment.