Are you really using a date format in your system which is different from the default US-English MM/DD/YYYY date format or did you just type a special date format here to tell us what's happening?
In Quicken please check Edit / Preferences / Notify that all settings on this panel (except the one about reconcile reports which is optional) are enabled. That should prevent most accidental register transaction changes.
1) Are you Syncing to Mobile/Web? AND 2) do you actually use the Mobile app on your smartphone or the Quicken on the Web browser-based app?
The Sync process has been suspected of a number of weird happenings with Quicken. Could your problem be caused by Sync?
If you answer "yes" to the 1st question and "no" to the 2nd question, turn Sync off completely. You don't need it. The Sync process should only be used in conjunction with the Quicken app for Smartphones or the Quicken on the Web browser app. It cannot be used as a replacement for backing up and restoring your Quicken data file.
If you answer "yes" to both questions, try to Reset Sync so that the current cloud sync file is replaced with a fresh complete copy
Do note that a full Sync process may take many minutes to complete.
How to reset Sync to Cloud
How to turn off Sync to Cloud for the current data file
@CynthiaF - Are you using the US version of Quicken or are you using the Canada version of Quicken. Your profile shows the Canada version. That date format you used (YYYY-MM-DD) is the international date format that is commonly used in Canada, right?
