Hey because my account is less than 3 days old it won't let me post in clubs yet. but French is almost a 1st language for me, although I've begun to forget a great deal as well. I should be able to help with your assignment.
"J'écrirai dans ce journal depuis le quinze jour suivant." is close, but should say "Je vais écrire dans ce journal pour les quinze prochains jours ."