Are you sure you got the times right? Bc I think the times listed in the bradyguide and most other guides is wrong. The time I have for Millie is 12:00 to 22:00 in Prickle Pine.
If you have the correct times, then you have the girlfriend glitch. This is a really common problem/glitch. For some unknown reason (hence, it's a glitch), sometimes girlfriends just stop appearing at home when they're suppossed to be at home. In fact, they never show up.
There are lots of different methods to try and get rid of the glitch, but there's no sure way to get rid of it. In fact, I've had the girlfriend glitch for about 40 in-game hours, and no matter what I do, it never goes away.
There aren't any faqs that can help you with this, but I'll just quote part of the girlfriend faq on gamefaqs, by bloodstorm:
"Q.) I went to a girl's home during the time she was supposed to be there, but she never showed up. What's wrong?
A.) After going out on a date with a girl, she usually isn't available for another date until about a day later (often she'll become available at midnight, if that's one of her available times).
However, it often happens that a girl will stop being available no matter how long you wait. One theory about why this happens is because you no longer meet the requirements to date the girl. To get the girl available for a date again, you need to get your appearance up to her standards. Also, she may not become available for a day or two after you get back up to her standards, so be patient.
It may be that the above doesn't seem to work, or that it's Denise or Millie that are not available for dating during their normal times. I'm not sure exactly what causes this, so that means I'm also not sure how exactly to fix
it. What seems to work best is trying to change your appearance, such as changing clothes, getting a new haircut, and maybe getting some new tattoos. Then you have to wait for a few ingame days before the girl will be available for dating again. If that still doesn't work, try doing a few missions if you have any left. They always seem to become available again at some point, it's
just hard to tell what exactly brings them back."
When or if Millie becomes available for dating again, remember that you don't have to date her to get the keycard. You can just kill her, and you can still get the keycard. Of course, you can't date her after that.