To be honest, that's a good question, Dana...
As I understand it, this pattern is still issued in the Japanese navy to this day.
With some Japanese military-pattern edged weapons (e.g., Type 30 bayonets and Type 95 NCO swords), arsenal marks, serial numbers, and/or acceptance stamps can sometimes be used to narrow down a date range with varying degrees of success. There should be arsenal marks or acceptance stamps on the crossguard of your dirk.
Another clue may be found in the same grips. Is the grip of your exmaple made with real or synthetic same? If synthetic, it dates to late-WWII or later.