Make sure you are using the datepicker pop-up to select dates when creating sheets/tasks so that the date is saved in the format required (dates will be translated on the front-end to the format you set in your regular WordPress settings, but they need to be entered in a very specific way). It’s possible to type in dates instead of using the date-picker, so if you did that, but used the wrong format, the date will be interpreted wrong.
Or, perhaps you changed the timezone on your WordPress site (or server?) after those tasks were created??
The settings for expired tasks will determine if you want the plugin to automatically delete signups that are expired (past date). If you have a lot of signups on a regular basis, checking this will keep the size of the signups table reasonable by deleting old entries from the table. However, if you want to keep a record of all signups (on the admin side, with the view expired option), then you do NOT want to have it set to automatically delete the old ones since once they are gone you can’t get them back.
Since this was something unique to your site, and there are no reports of similar issues (and I could not recreate the issue on my test sites), I’m marking this as resolved.