I have the exact same problem.  Last December 31, 2020 I was doing the cheque for the rent dated January 1, 2021 and it kept giving me an error message.  So I dated it December 31, 2020.   Today when I advanced the date, it read must be between 01/01/80 and 01/02/21, however the date is 01/02/21 and the error keeps flashing and I could not do any reports.   Similarly, backing the date to December 31, 2020 is all fine.  

We will need a solution to this.  Anyone in Sage??

I have the exact same problem.  Last December 31, 2020 I was doing the cheque for the rent dated January 1, 2021 and it kept giving me an error message.  So I dated it December 31, 2020.   Today when I advanced the date, it read must be between 01/01/80 and 01/02/21, however the date is 01/02/21 and the error keeps flashing and I could not do any reports.   Similarly, backing the date to December 31, 2020 is all fine.  

We will need a solution to this.  Anyone in Sage??

Bank reconciliation works for 2021.  Any kind of report doesn't. 

I tried to start a new year, backed-up the date beforehand.  Even in the new fiscal year 2021 does not work, so I restored from backup.  No need to try that option, it didn't work. 

Aaron,

You can't post-date Payables entries past the Fiscal or Calendar year ends without starting the new year first.  That is why you got the original message when you tried to use Jan 1/21 when your session date was still in 2020.

For the 2021 issues, the same questions I posted for Dimery may help sort out your problem.  I've edited the original post I sent and added a new question.

Also can you activate four-digit years in Sage 50 and let us know if the century is still 20xx in the messages?