Why do two different service dates matter, and which one decides my pension?

Two clocks, started for different reasons

DIEMS — Date of Initial Entry to Military Service — marks the first time you entered any component. It is the date the retirement system rules are applied against, and it is meant never to change, including across breaks in service and moves between components.

PEBD — Pay Entry Base Date — is the pay clock. It reflects creditable service for pay purposes and can legitimately differ from DIEMS where there were breaks in service.

They are close together for most people and far apart for anyone with a prior enlistment, an academy or ROTC entry, or a gap. That closeness is what makes the difference easy to miss, and the people for whom the difference is large are exactly the people most likely to have an error.

Why the DIEMS error is the more serious one

A PEBD error costs money continuously and is visible: your pay does not match your years, and the discrepancy shows up on an LES you look at every month.

A DIEMS error is quiet. It does not change any figure you see day to day. It determines which retirement system you are in, and you may not discover the mismatch until you are planning a retirement built on the wrong multiplier — or until you find you were never receiving TSP matching you assumed you had.

Check both now, not later

Both are printed on your records. Confirm them against your own documentation — enlistment contract, commissioning orders, prior-service records — early, while the supporting paperwork is still easy to lay hands on.

Correcting either is an administrative process, and it is dramatically easier in year two than in year eighteen. The evidence you need still exists at both points; what changes is how long it takes you to find it and how many downstream records have since been built on the wrong date.