I found that Windows 7 uses "Date" instead of "Date modified" in some folders, such as the "Pictures" folder.
So initially, I thought they are the same, but until later, when I know some files were modified last night, but if I arrange the files by "Date", I don't see those files... (there are close to 100 files in this folder, and the view is "Details" -- as a list)
So I thought this is a bug with Windows 7, with all those files missing. Refreshing the folder didn't work. Until finally, it seems that the "Date" is the "Date created"... so that's why the modified files didn't show up near the top of the list. The creation date was much earlier.
This caused quite some confusion. I wonder if there is any way to make all folders show "Date modified" instead of "Date" instead -- one simple step for all folders.