Photos and videos are time and date stamped by your camera when they are taken. Sometimes when the photos/videos are not loaded onto the date we expect them, they may show up on the top of your Gallery as if they were created today.
You can correct the date of the photo/video easily. Just click on the picture and then click on the date under it to edit.
Please look through your site to see if the photos/videos you are looking for are on a different date. Also, check your camera settings to make sure they are correct.ff