Eintrag = entry
Beitrag = contribution
Both nouns are related with verbs:
eintragen
This means to write a new item into a list.
Wer mit dem Bus zum Treffen fahren will, muss sich in diese Liste eintragen.
Who wants to travel with the bus to the meeting has to enter his name in this list.
So an »Eintrag« is an entry in a list or in something that is similar to a list. (i.e. a logfile or a class book)
beitragen
This means you add your part to something bigger.
Ich will mit meiner Spende dazu beitragen, dass es anderen besser geht.
With my donation I want to make a contribution in order to help others.
So a »Beitrag« is what someone adds. Together with »Beiträge« from others it forms something that is bigger than what one person could have made alone.
Since an entry in a blog is a contribution, you can use both words, and so all of this four possibilities are correct and understandable:
- "Eintrag vom 21.01.2014 12:12", "Eintrag von Herr Mustermann"
- "Eintrag vom 21.01.2014 12:12", "Beitrag von Herr Mustermann"
- "Beitrag vom 21.01.2014 12:12", "Eintrag von Herr Mustermann"
- "Beitrag vom 21.01.2014 12:12", "Beitrag von Herr Mustermann"
But because the first part (»xxx vom 21.01.2014 12:12«) has the focus on the list-idea, and because the second part (»xxx von Herr Mustermann«) deals more with Mr. Mustermanns contribution, #2 is the choice you should prefer.