A Time Calculation Problem: 3yrs or 3yrs and 1 day? Suppose, a person A has his birthday on 30 Sept 1994 and another person B has his birthday on 30 Sept 1997, then which of the following statement would be correct and why?

The difference between the ages of the two persons is 


Three years correct to the number of Days
Three years and 1 day correct to the number of Days (because of 29th Feb 1996 that comes in between)?
It rather depends what you mean by a year, which is approximately 365.25 days. This article
 gives a huge range of options with more precision and specificity.
Really you should be using SI Units, they are designed to stop problems like this occurring.