Is there any online food diaries that let you update and post things even if you posted them a late, like the next day if I forgot that I ate an apple as a snack I can add it in to the previous day?

MyNetDiary certainly does this. You can also pre-enter things for future days if you want.

livestrong.com does

Thank you guys very much.

My fitness pal is awesome. It does that and has made a huge difference with counting calories.

FitDay does too

I chose to use this one by the way, and I quite enjoy it. Thank you again.
Turns out I have been undereating. Oops. heh, heh.