Dates are cyclical except for the year, which increments by 1. So on 1/9/1940 you get 69. Therefore you can deincrement the month or day to get 68 again. In this case 1/8/1940 = 68. When you're adding up the same numbers every year, and predictably adding 1 every time, then it's easy to see how this can pop up every year. It's a consequence of adding a small set of numbers together.

As for finding specific dates, if you know one you can figure out the others. If 1/8/1940 is one then 2/7/1940 would be another. 3/6/1940. You can keep going from here. It happens a lot during a year. The only time it won't happen is when the year itself gets to be too large. 20 + 22 = 42, leaving you only 26 to work with. In 3022 you'll only have 16. 4022, only 6. Eventually you won't be able to add up dates and get 68 at all. So if numerology is real and there's some pattern that shows war breaking out ever date that adds up to 68, then we'll have world peace by 4028.