The selected Leap Year convention determines the number of days assumed in a leap year. The conventions supplied by Resolution are defined as follows:

Convention

Definition

.

Actual

The number of days in a leap year is 366. The number of days in a non-leap year is 365.

365

The number of days in every year is assumed to be 365, even in a leap year.

365 ISDA

The number of days in a leap year is 366, and the number of days in a non-leap year is 365. However the DayCount fraction (the proportion of a year) is calculated using the following formula:

Result = Days_ly /366 + Days_ny / 365

Days_ly = Actual number of days in period that fall in a leap year.
Days_ny = Actual number of days in period that fall in a normal year.

This contrasts the Actual/365 DayCount fraction which is determined by dividing the actual number of days in the period (regardless if there is a leap year) by 365.