Long time Quicken user. This appears to be a bug and is very unintuitive. I don't recall this working like this across 20 years. Why is this happening? Customer support said it's a feature or the way it works, but there's no explanation. To me it's an ongoing annoyance.
I created a scheduled transaction to have me manually move money from one quicken account to another. I missed the date due to vacation, but this also happens if I purposely decide to delay.
The transaction is now listed with the red clock reminder. I then open up the transaction, where I can edit the amount and click Mark as Deposited but I can update the date later than the originally scheduled transaction. If I enter today's date, which is beyond the original, then I get the Invalid Instance Date error. I have to first Mark as Deposited and the reopen the entry to edit the date. This doesn't make sense.