player.getLastTimeOnline() seems to only work for currently online players

I have not used player.getLastTimeOnline() so far, but I imagined a solution!
It is possible to access the World-Database to fetch data from all players.

WorldDatabase database = getWorldDatabase();
    try(ResultSet resultSet = database.executeQuery("SELECT LastTimeOnline FROM Player WHERE Name LIKE 'Name';"))
    {
      if(resultSet.next()) {
        long lastTimeOnline = resultSet.getLong("LastTimeOnline");
        Timestamp stamp = new Timestamp(lastTimeOnline);
        Date      date  = new Date(stamp.getTime());
        System.out.println("last time online = " + date);
      }
    }
    catch(SQLException ex)
    {
      ex.printStackTrace();
    }

I hope this could help you out!

Upps, my bad! Now I understood! It's a bug, not a feature!

...or with some other words:

It's indeed a trap I have to admit the name of this function may be misleading. Actually it is supposed to return the timestamp of the date when the player was previously online (i.e. if I play on a server on March 1st, then take a break and join the server on April 10th again, this method will return the timestamp of the 1st of March).
As you suspected, the server.getPlayer() functions only work with online player, so this method isn't accessible for offline players. I'm not sure if it makes sense to cover offline players by these methods as well (since most funtions of the Player object won't make much sense for offline players)... we'll have to think about that
The old Lua API provided a function to get some player information from the database, maybe we will add this function to the new API as well. Otherwise quering the database, as mentioned by @Machete would do the trick (actually a "getPlayerInformation()" function would do exactly the same^^)