To calculate a positive year difference and the corresponding month difference, a. Subtract the earlier year from the later year to get the year difference. b. Subtract the month associated with the earlier year from the month associated with the later year. The month difference can be negative (see examples in the table below). In sample 2, the month difference is negative. The difference, 3 years and -2 months, means it would be 3-year difference if this were 2 months later. A date difference should not contain a negative month difference. A negative can be adjusted by regrouping one-year difference into 12-month difference. 2. To calculate a date difference between two dates that are in the same year, the year difference is always zero. The month difference should be calculated as a non-negative value.
details is at part IV in the file I upload