"You can change the date and time taken of photos and videos using Windows Live Photo Gallery. Open the file in the gallery, right click on it and click change time taken. You can also do it for multiple files."
Helency got it mostly correct here. Open the file in the gallery, but do not right click the image. This brings you to the same edit properties screen that doesn't let you change the time. Instead, click the button in the upper left corner 'edit, organize, or share'. This will bring you to a new screen, which will hopefully already the file location of the photo you want to edit already open. If not, you can use the scroll pane on the left to find the file folder. Once you have that folder open, use the main pane to find the photo. Hover over it and check the box that appears on the upper left corner of the thumbnail image. Once you have done that, click on the 'edit' tab at the top of the screen. The 'adjust time' option is on the bottom of the third column of options. From there it is easy to change the time.
PS for Microsoft - This is a bass ackward and horrible way to have to change the time, especially since the time stamp is used to organize photos in folders. We ought to be able to change the time in the properties screen that pops up when we right click an image. I had to expend my monthly allowance of screaming obscenities trying to figure out how to do something as simple and fundamental as changing the time on my photos. Please change this issue and lower your customers blood pressure. Thank you.