This is a promise with a capital P. Not just a simple "I promise," but a solemn, binding agreement. Think "Luma at Bagong Tipan" (Old and New Testament). On a less serious note, it's the old-school word for a date or appointment. "May tipan kami mamaya" sounds way more dramatic than "We have a date later." It's like you're meeting for a secret treaty negotiation, not just for milk tea.