The main reason you're seeing mostly eight-digit dates is because the world just spent billions of dollars a few years ago dealing with the problems inherent in the format you are proposing. That caveat out of the way, there are a number of ways to deal with your issue.
- There are Date and Time modules on CPAN. Some of them are particularly suited to managing date ranges.
- You could play with times by adding seconds to the epoch clock, but you'd need to be aware of many issues the date and time modules handle for you.
- You can use the unary increment operator (++) on a value of 80623 and format the printing how you want, but you'd have to roll over the months and days with a nasty collection of case or if statements. You'd still also need to handle many issues the date and time modules handle for you.
- There are probably even worse ways to handle this, and we could probably have a contest to see how perverse a working solution could be.
If you search the PM site, you'll probably find some great comparisons and advice about date and time modules as well. One should do what you need.
Update: s/data/date/ in the second list item.