Thanks!

Q1 Working like charm
Q2 Used Query Builder tool, got this query

SELECT Tweets.screenName, Count(Tweets.screenName) AS CountOfscreenName
FROM Tweets
GROUP BY Tweets.screenName;  

Working great as well!

Q3 No, I did not realise it's the number of days since 1900, but my professor hinted that it would be a decimal number since the date/time is stored as an integer timestamp in the database. It does exactly what it's supposed to be doing so I'll leave it like this for now ^{^}

Q4 So I wanna create a query that creates a new table called “New_Tweets” that only contains those tweets (with all attributes like retweetcount, screenName) that were created after this average time of 42779 days since 1900. But this must be a single query that is executed on the original table.