User field image deleted next day
Are you sure the files are not temporary? It does sound like the files are being deleted because they are not yet permanent as Jayendra suggests.

Are you handling the file uploads yourself? Like through a custom form, because I've forgotten to save files permanently before, and I remember wasting a lot of time on it figuring it out :)

The temp files are cleaned with cron I believe, so I find it weird that altering your computer date would trigger the issue. They might however also be automatically cleaned up when you try to render an image that's not yet permanent, and that has passed it's 'lifetime'.