The U.S. Army uses a soldier's "pay entry basic date" (PEBD) to determine creditable service periods, which it uses to determine your eligibility for pay and benefits. It is essentially the day you first came on duty, minus any breaks in service, from which they count how long you've served. The Navy and Marines call it "pay entry base date," the Air Force simply calls it "pay date," and the Defense Department uses the general term "basic pay date." No matter your branch of service, Department of Defense regulations calculate the date the same way.

Compute initial basic pay date. This is the time you've served since your most recent service entry date with no breaks. For enlisted, use date of enlistment. For officers, use date of acceptance of commission; if you attended a service academy, this is your date of graduation. Count the number of days since that date and add one to count today. For example, March 1st, 2010 to March 22, 2010 is 22 days.

Determine creditable service for any previous service periods by counting the days between start and end dates of those service periods. Be sure to add one day to non-consecutive periods of service to account for inclusive days.

Add the the number of days from Step 1 to the days from Step 2 to get a total of your days of service.

Convert the days from Step 3 to years, months and days, using 30-day months for your calculations. For example, 364 days is exactly 12 months and 4 days.

Subtract the time from Step 4 from today to get your effective PEBD. For example, Jan. 2, 2010 minus two years, four months, and one day yields a PEBD of Sept. 1, 2007.