Help creating a scoreboard that shows how many mobs a player has killed and how many days have passed?

I would like help with the scoreboard system, I don't understand much and the forums I found here don't take me very far, I would just like to show how many of a certain entity the player killed and how many days passed in the game, I don't know if it would be foolish to assume that it would be easy because the game itself has this information, being the amount of days seen by f3 and mobs through statics