multiplyDuration doesn't handle fractions of 1

[justincy]

GedcomXDate.multiplyDuration('1Y1M1D', .5) will do nothing. This is because it handles the parts completely independently and rounds. So half of 1Y is .5Y which gets rounded back to 1Y.

I don't know how to handle this. What is half of one month? It depends on which month. Same with years, some have more days than others. I'll investigate how other date libraries deal with this. It's straightforward (though not fun) when you have some reference point so that you know which year or month you're dealing with, but you don't get that with generic durations.

[justincy]

Google says that a month is 30.4368 days.

[justincy]

PHP has a DateInterval but it's only used for adding to or subtracting from Dates, you can't perform any operations on the interval itself.

Moment.js has Durations but you can only add or subtract a specific amount of time, you can't multiply or divide.

[justincy]

Moment.js also defines months (for the purposes of durations) as 30 days and years as 365 days. If we took the same approach then it would be trivial to support fractions with multiplying durations.

If we don't want to do that then we should explicitly disallow multiplication by fractions on generic durations and instead add a method to Simple dates that allows adding and multiplying. But even then it might not solve all the problems so I recommend that we take the same approach as Moment.js. Date math is inherently imperfect so we might as well make some basic assumptions which simplify the process and make it deterministic.