Function Dirty Price within with keys returns the of the referenced bond(s) at a given settlement date Tˢ of an assumed bond trade transaction.
There exist 3 distinct methods for obtaining the dirty price:
1) From a given as of the date Tˢ
2) From a given flat yield as of the date Tˢ that acts as the discount zero rate for all cash flows.
3) From a given discounting yield curve that implies the discount factors to be used for discounting the various cash flows.
The last case takes no input for Tˢ, which is regarded equal to the bond's settlement date (see below) and works as follows:
All bonds cash flows occurring after Tˢ are discounted down to today and then their sum is then divided by the discount factor between today and the settlement date.
In all other cases, if Tˢ is not explicitly given, it will be set to the bond settlement date as implied by a trade transaction assumed to occur on the valuation date T₀ (typically today).