ISDA assigns a very specific meaning to "Calculation Period" (Date1 to Date3) confusion can ensue. Reading the original French, the period referred to is Date1 to Date2, not Date1 to Date3. The original French version of the convention contained no specific rules for counting back the years. A later ISDA paper added an additional rule: "When counting backwards for this purpose, if the last day of the relevant period is 28 February, the full year should be counted back to the previous 28 February unless 29 February exists, in which case, 29 February should be used". No source can be found explaining the appearance or rationale of the extra rule. The table below compares the later ISDA count back rule to a simple count back rule (which would have been implied by the original French) for one of the few cases where they differ. The simple rule illustrated here is based on subtraction of n years from Date2, where subtracting whole years from a date goes back to the same day-of-month, except if starting on 29 February and going back to a non-leap year then 28 February results. Sources: "Definitions communes a plusieurs additifs techniques", by the Association Francaise des Banques in September 1994. FBF Master Agreement for Financial Transactions, Supplement to the Derivatives Annex, Edition 2004, section 7i. Actual/Actual comparison, EMU and Market Conventions: Recent Developments. ISDA Actual/Actual paper, 1999. 1/1 This is used for inflation instruments and divides the overall 4-year period distributing the additional day across all 4 years i.e. giving 365.25 days to each year. Sources: ISDA 2006 Section 4.16(a). FBF Master Agreement for Financial Transactions, Supplement to the Derivatives Annex, Edition 2004, section 7a. Discussion Comparison of 30/360 and Actual methods The 30/360 methods assume every month has 30 days and each year has 360 days. The 30/360 calculation is listed on standard loan constant charts and is now typically used by a calculator or computer in determining mortgage payments. This method of treating a month as 30 days and a year as 360 days was originally devised for its ease of calculation by hand compared with the actual days between two dates. Because 360 is highly factorable, payment frequencies of semi-annual and quarterly and monthly will be 180, 90, and 30 days of a 360-day year, meaning the payment amount will not change between payment periods. The Actual/360 method calls for the borrower for