Dear Debbie: Great question. First, if he were to omit all dates from his resume then assumptions would be far worse than reality. The key would be to decide how much of his 20-year career to include. If he is pursuing more senior-level roles then presenting 20 years might be just perfect. If however he is pursuing roles of a more mid-level, then perhaps only dating positions held over the course of the past 10-15 years may be more prudent. To do this without being misleading, date titles instead of tenure with the military. For instance, if he held 4 roles over the course of his career, perhaps only include the last 2 or 3 in order to only present about 10-15 years of his career. You can always “byline” the earlier role(s) with a brief italics statement at the end of his professional experience section. This statement would simply state, “Additional experience with the military in foundational leadership roles” or something related to what enhances his candidacy today. The key to this byline is not to date it so that you avoid unnecessarily aging his candidacy.
I imagine however, that based on a 20-year track record of success in the aviation field, he is somewhat of a subject matter expert (SME) in his field. When you are in this position, you are able to present more of your career to “back up” your subject mastery. I’d probably focus less on how many years to present and more on differentiating his candidacy through the uniqueness of his experiences, education, and key contributions. I am confident that if he does this his job search will be a success.