I would say don't let the calendar dictate when you should get engaged. One of the keys to having a successful marriage is making sure (you are both on the same page and want the same things). Every marriage will have it's challenges but at the very least it should start off with (both people "wanting" to get married). In order for him/her to be "the one" they must also see you as being "the one". That's not something you can demand from a person.
It takes more time to truly to get to know someone then most people are willing to invest.
Dating essentially is the "due diligence" phase of a relationship where you want to find out how your mate feels about everything you consider important. Opposites may attract in the short run but it's like that attracts like in the long run.
Too many people choose to ignore the clues/signs of potential problems when they are "determined to get married". The wedding is just one day. Nothing "magical" happens after saying "I do".
You want to make sure the two of you (agree on the important things in life) as well as on the strategy for obtaining them. Life is too short to spend time trying to change water into wine or fit square pegs into round holes.