public long Subtract(LocalInstant minuendInstant, LocalInstant subtrahendInstant)
{
int minuendYear = calculator.GetYear(minuendInstant);
int subtrahendYear = calculator.GetYear(subtrahendInstant);
int minuendMonth = calculator.GetMonthOfYear(minuendInstant);
int subtrahendMonth = calculator.GetMonthOfYear(subtrahendInstant);
int diff = (minuendYear - subtrahendYear) * calculator.MonthsInYear + minuendMonth - subtrahendMonth;
// If we just add the difference in months to subtrahendInstant, what do we get?
LocalInstant simpleAddition = Add(subtrahendInstant, diff);
if (subtrahendInstant <= minuendInstant)
{
// Moving forward: if the result of the simple addition is before or equal to the minuend,
// we're done. Otherwise, rewind a month because we've overshot.
return(simpleAddition <= minuendInstant ? diff : diff - 1);
}
else
{
// Moving backward: if the result of the simple addition (of a non-positive number)
// is after or equal to the minuend, we're done. Otherwise, increment by a month because
// we've overshot backwards.
return(simpleAddition >= minuendInstant ? diff : diff + 1);
}
}
internal int GetMonthOfYear(LocalInstant localInstant)
{
return(yearMonthDayCalculator.GetMonthOfYear(localInstant));
}