I'm wary of making commitments that I might not be able to fulfill, so I don't make them if put on the spot. I'll say a provisional "yes" and let you know later when I've had a chance to look at my diary, I'll give you a more definite answer, and once I've given it I'll stick to it.
I will easily reschedule though, if it turns out to be a better idea for all concerned - if asked to I don't mind (and secretly welcome the upheaval lol), and I'll initiate a rescheduling if it's obvious that people are clinging to the prior arrangement purely for the sake of avoiding change.
I'm almost never late for anything unless something out of my control happens like freak traffic conditions or family illness, etc. and just because I prefer not to get up early in the morning doesn't mean I can't do it if I have to. But I don't really mind when other people are late - I've a friend that I regularly do lunch with and he's been at least 15 minutes late every single time for the last 8 years, and I just find it amusing, though I know a lot of people who tell me they'd have told him to go to hell by the third time he did it.
This isn't my first nature - I've learned to be more dependable because of the big stigma and assumptions about my character (uncharitable and untrue ones) that were often made, quite hurtfully, back in my teens and early 20's, I guess I set myself the task of proving everyone wrong to myself, and then to them. I sorta see these things as a challenge, first to commit and secondly to follow through. I developed a sorta code of honor to hold myself to, so I can't easily worm out of things with intellectual justifications. But also raising kids on my own gave me the kick up the arse too that I needed, cos there's no way I could let them down or break promises to them, not when I grew up with my mom doing that to me all the time and knowing how much it hurts. Also cos their schedules were more rigid when they were in school, I had to arrange mine around theirs and so everyone that wanted to deal with me had to arrange around my schedule, even though by nature, I don't like schedules.
I don't keep a diary for any more than two months in advance. I just won't commit to anything that far in advance, unless it's something you have to pay up front for like a holiday or getting tickets for a show that sells out quickly, etc. If I've got too many things on the calendar for too far ahead, I feel completely choked and imprisoned by it all, and like I can't just live my life until they're over with.