Present continuous for future plans
Sorry, I can't join you for lunch. I will meet my writing supervisor to discuss my dissertation.
For events in the future that are..
- difficult to re-schedule
- circled on a calendar
Sorry, I can't join you for lunch. I'm meeting my writing supervisor to discuss my dissertation.
I know, the meeting is due to take place in the future, but when an arrangement has been made with someone else and is difficult to change, use present continuous tense, especially when you're excusing yourself from some other offer.
Sorry but I have to go now. I'm teaching a class in 10 minutes!