What's the best way of using joda time to manipulate monthly data?

Question

I'm trying to work with a timeseries of monthly data. That is, there is a value for each month, and no specific date (or time) associated with it.

I could have arbitrarily fixed the day, time at some constant (eg midnight on first of the month) then used a java.util.date or joda DateTime but decided to try and do this within the joda type system.

My first step was to create a YearMonth class which extends AbstractPartial (by analogy to LocalDate). I've now reached the next step of trying to increment a YearMonth by a number of months. I could write the code myself (adding % 12) but I'd rather use a Joda style way of doing this.

What code should I write, and where should it live so I can for example add a 13 month or 2 year Period to my YearMonth class? Or am I going down a bad track?

Answer 1

Year/Month math has to be done as follows:

ym = (year*12+month)

Do your math on ym.  Add months or years, or whatever

year = ym / 12
month = ym % 12

There's no real alternative. This is provably correct and no alternative will actually be simpler. Some languages (like Python) have a divmod function that combines the last two lines of code, but that doesn't change the essence.

You have units (months) that you will display to people as year-month. It might as well be Yards-Feet or Hours-Minutes or any other simple conversion to a two-part base.

Answer 2

The joda-time svn has a YearMonth class, ready for v2.0.

Answer 3

DateTime currentInstant = new DateTime();

// add a month

currentInstant = currentInstant.plusMonths(1);

// add 2Y + 13M => 3Y 1M

Period incPeriod = new Period().plusYears(2).plusMonths(13);

currentInstant = currentInstant.plus(incPeriod);