Your understanding of date ranges, once your system date is 2000 or beyond, is correct. To give you the complete picture, if your system date is 1999 or earlier, then 2 digit dates are : 10-99 refers to 1910 to 1999, and 00-09 refers to 2000 to 2009.

Also, to be clear, once a date is entered, when it is posted it is stored internally as that precise date and nothing alters it (unless you Adjust Rental lines, Return it, etc at which point you re-enter the dates on the screen and you are back to the 2-digit date scenario as above). The point is, that once a contract is posted, even if back in 1980, it will continue to be billed properly.

So the only tricky situations are for contracts dated before 1990. You will be able to return them without trouble. Its Outdate will say 89 and in this case will mean 1989 (a sleight of hand) and because it can't be edited it will remain as 1989. The In-date (Due Date) will become Today's Date, since you are bringing it back today.

The two areas where things get tricky is the Adjust Rental Lines, and the Front Counter Header screen's Start Date and End Date. There are ways to handle these situations and you can pursue this further if needed.