How is seniority between NCOs of the same rank determined?

It's date of rank. When both dates of rank are the same, then total active federal service (BASD).
It's in AR 600-20 Army Command Policy

TIG takes precedence over TIS. If those are the same then it goes to TIS. And as a kicker, of both of those are the same then DOB is the determining factor. Reference AR 600-20 The Army Command Policy.

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