What it is
Your creditable years of service for pay, derived from your Pay Entry Base Date. The pay table columns step at 2, 3, 4, 6, 8, 10, 12, 14, 16, 18, 20 and beyond, so crossing one of those marks should bump your Basic Pay that month.
What goes wrong
Know your next step date. The month you cross a pay table column, confirm the raise appeared. Longevity raises going missing at the step date is a quiet, recurring error.