So can we assume that wherever the future is implied by a specific time (like "next year", "when I visit you", "in 2027"), Dutch uses the present tense for the verb?
Doesn't English also accept present tense when there is an adverbial indication of future?
Yes, informally, we say things such as, "I travel to Mongolia next month" and "He is 100 next birthday".
I find it is better to avoid informal English. Sometimes Duolingo accepts it, but sometimes it doesn't.
This is quirky in English. I might say, "I see you again next week," in the narrow context of reminding someone of our next appointment together, but if I'm talking to a friend or acquaintance in some general sense, I would use some future tense of the verb: "I will see you again next week."
Well, I am all for as much learning as possible. But here the focus should be on Dutch. The acceptance of present tense in these settings is by no means rude or slang. I would in fact argue that - as quite a number of European languages fully accept it already - it is well on its way into formal English as no language is static.