What do you eat every day...
I bought the PC version- which just has more stuff that I can't think of either!
I did my chart on my profile using it, plus some help from Paint Shop Pro. I always have Fitday opened in my task bar so I don't forget to input everything. You can see how many calories you've consumed for the last week, month, etc. Same with every nutrient (helpful with trying to get more calcium in) and the Weight goal part is the only chart on weight loss that I like!
I definitely think it's worth buying it, no matter how far out you are. However, play around with the free online version first and see if you think you'll use it.
And yes, you can put in past numbers, but that will get old real fast. When you average things out, it doesn't count the days where there was no input, so it's not going to mess up your totals.