I'm thinking your session date is currently May 31st, if so, change it to today

I think Roger is correct.  You can change it from the Maintenance menu.  Or you can have the session date window show up when you start the file by selecting Setup, User Preferences, View, Show Change Session Date at Startup.

Change it to whatever TODAY is each time you open the file.  I just hit the down arrow to do that and then hit Enter.

Okay, I will try this and see if works. I had set the fiscal year date to May 31st when I opened the company in the database, perhaps it was going from this?

We are not talking about the fiscal start or end dates here.  We are talking about which date Sage 50 thinks it is.  That date is used to give you the date warning messages you are getting.  If you post into the future past the session date (as it appears you are doing by posting Sep entries with a Session date of May), it will warning you that you are posting a future dated entry or if you post to the previous month, it will tell you that as well.

I never really paid much attention to the first Session date but I assume it would be the fiscal start date.  I always change it daily.  When I get to the end of the fiscal year, I might hold it on the last day for a couple of days, but then move to the new year once I have time to get all my billing done for the month and get all my backups done.

You can also change the number of days it warns on, under:

In Setup | Settings | System   are these settings:

[ X ] Allow transactions in the future (beyond the session date)

   [ X ] Warn if transactions are [      ]  days in the future

If you commonly work on information that is dated 'in the future', then you may want to change number of days for warning, or deselect it entirely

You can also change the number of days it warns on, under:

In Setup | Settings | System   are these settings:

[ X ] Allow transactions in the future (beyond the session date)

   [ X ] Warn if transactions are [      ]  days in the future

If you commonly work on information that is dated 'in the future', then you may want to change number of days for warning, or deselect it entirely

I'm actually dealing with dates in the past due to the company starting from the ground up with office admin responsibilities. Will this also work for days in the past? 

Sage allows you record transactions for 2 fiscal years (current fiscal year and previous fiscal year)

note, you can report on / view transactions for all other years in the company file

so if your FY ends Dec 31st, today, you can record back to Jan 1, 2013

KRAT said:

...Will this also work for days in the past? 

You do not indicate what "THIS" is so I am assuming it is the warning message about future transactions.  Because it is only for future transactions, past one's (those in previous months) are always flagged and will tell you the reporting will be altered when you post, but it will let you post, as Roger says, within the current or previous fiscal years.

When I am playing catchup with a client file, I usually do a gross sort of everything by month (not always perfectly sorted but at least most of it sorted), set the session date for the end of that first month and then post them.  Then move to the next month by changing the session date to the end of the month.  If you get any warnings (please read them all before you hit the Yes button), then you will know you have miss sorted in some way or mistyped in some way and can take appropriate action.