As Asai has stated, the dates are hardcoded. You, as the player may not necessarily get an offer at that prescribed date, but they are showing up elsewhere, to be sure. This is why I never wait for them, anymore. You can have muskets (not just the arquebus) long before any other clan by accepting the first Portuguese offer and running with it. If your concern is for rebelliousness, then plan ahead and have shinobi ready to placate the populace. If you prefer having monks (although you can have both monks and guns) then I suppose you can wait for the Dutch.

I've stop using monks a long while back, except if I capture Kii or Kaga with fully developed dojo's, or if I'm playing Mori. The added benefit of having both cathedral and trading post income far outweighs the impact of monks, IMHO. I've had several campaigns where I wouldn't have survived without those two sources of income.

Just my 2koku on the matter;)