Proposing drinks for tonight and being offered a slot three weeks out is a standard early culture shock, and it reads as a brush-off to anyone from a more spontaneous social culture. The agenda (diary) is a stricter instrument here than the word suggests, which is what makes the misreading so easy.
Because the agenda is treated as sacred here — calendars fill up fast, and proposing something last-minute can genuinely read as inconsiderate of someone else's already-planned time, not as fun and easygoing. This isn't coldness; it's the flip side of reliability. Once something's in the agenda, it happens — no flaking, no "let's see how the day goes." Dutch social life, work meetings, even family visits often get scheduled a month or more out, sometimes literally with a shared calendar invite. The adaptation is simple: stop waiting for the mood to strike and start proposing specific dates two to four weeks ahead, the way locals do with each other. It also means once you're in someone's agenda, you're genuinely in — that dinner in three weeks isn't a vague maybe, it's happening. Spontaneous drinks still exist, mostly with very close friends or as an occasional exception, but as a default social mode it's rare. Once you stop reading the planning as distance and start using it, it actually makes Dutch social life easier to navigate than more spontaneous cultures — you always know where you stand.