PerpetualPreferredCalculatorgetInterestAccrualDate Method
Get the date on which interest begins to accrue for this settlement
to redemption period.
This method will return dated date if the settlement date is before
the first interest date, the coupon date just prior to the settlement
date, or the settlement date if it falls on a coupon payment date.
This method may not be applicable to all implementations, in which
case a null should be returned.