I think there is an inherent asymmetry to the problen. You can prove that something (a headline) exists when you do your message, but you cannot easily prove th

I think there is an inherent asymmetry to the problen. You can prove that something (a headline) exists when you do your message, but you cannot easily prove that it does not yet exist. You could, theoretically, use something that one day will cease to exist to show that you signed your message before that point. However, digital things (information) normally do not cease to exist. Assuming you don't want to involve any third party, things get difficult. To be honest I think there is always a way to make a message look older.