I tend to be confused about the dates of my husband's birthday - I know that it's one day away from my Mom's, but tend not to remember if it's before or after without asking. But we've only been married two years, so I'll get it straight eventually.
I think it does matter how long you've been together. In the early years, you're still adjusting to each other's styles, and it can take a while for someone to whom birthdays really aren't important to get that their SO is different.
However, after a few years of spelling out that that it is important to you to have your SO remember and plan something for your birthday, I don't think it's unreasonable to expect them to program their computer to remind them as needed. At the same time, you need to pay attention to whether your own expectations are too great. If your spouse is someone who is normally loving and thoughtful, but who just isn't good at planning events or buying gifts without help, then you have to adjust your expectations - whether you want a surprise that's not all that great, or something really nice you help plan.
I've seen the reverse, though; a spouse who just will not accept the fact that their partner doesn't like celebrating birthdays and doesn't want any special notice when theirs comes around, and keeps planning surprise parties for them.
