Date in Period

Key Date in Period of function Period Counter in IRSrefers to a date D that defines the iᵗʰ accrual period [[Tᵢ₋₁,Tᵢ)) as the only unique period of the given swap's leg that contains that date.
Note, the leg consists of N consecutive accrual periods defined by the date sequence T⁰, T¹, T², ..., Tᴺ, as below:
[T⁰,T¹) , [T¹,T²) , ..., [Tᴺ⁻¹,Tᴺ)