We just migrated from another forum til SMF.
The old forum could show latest post from a time range - like "24 h", "48 h", "last week" etc. regardles of what you had seen since last login.
Is there a similar way to do do it in SMF?
At this time there is no default method that I know of to do this.
You would need to modify some of SMF's coding to do this.
Did you look at our mod site or Tips and Tricks (index) (http://www.simplemachines.org/community/index.php?topic=15899.0)?
Did you ask in features and request or codding discussion to see if anyone is willing to help you make this a mod?