For example I want to calculate a year fraction between the dates: 2011.03.31 and 2015.05.30.

There are 1521 days between the two dates, but 2012 was a leap year. So what should I use in the denominator? 366 or 365 if I want to use the ACT/ACT convention?

  • $\begingroup$ Number of days in 2012. $\endgroup$ – HelloWorld Aug 4 '16 at 15:52

As I know act/act you are using the sum of 2 fractions. One with 365 and one with 366 in the denominator. In your example the number of days in non-leap-years is 1155 and we have 366 days in leap-years. The resulting year fraction would be 1155/365 + 366/366 = 4.164

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    $\begingroup$ My way of doing it is like this (same answer of course), year by year: $275/365+366/366+365/365+365/365+150/365$ = 4.16438 $\endgroup$ – Alex C Aug 5 '16 at 0:27

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