How to convert month to other duration measurement types? - date

Question

For some duration-related calculations I need to convert values measured in "months" to other formats, such as years, days, or hours.
For example, what is the proper way to measure a month in terms of days? is it 30 days? or 30.4375 days? (365.25 / 12) and which format would be useful in which cases?
If you have any information on the casual/business use cases for such conversions it would be helpful too.

Answer 1

Unfortunately, there's really no single generally valid answer to your question.
If this is for business use, first check whether there are any existing relevant standards or business practices that define what a "month" means in your business context. If yes, you should follow that definition as closely as possible, however silly or awkward it may seem.
For casual use, the simplest solution is probably to pick any widely use date manipulation library and do whatever it does. The default behavior may not be perfect, but it's probably at least close to a fairly sensible compromise of the many contradictory expectations that users of such a library may have.
OK, but what if you insist on rolling your own solution? In that case, the first choice you should make is how you want to represent date / time values. There are at least two common choices:
The first option is to store dates / times using a simple linear count of fixed time units from a given epoch, such as Julian days or Unix timestamps. This provides a simple and compact date/time representation, makes comparing timestamps and simple date/time arithmetic (like adding n seconds to a time value) easy, and ensures that any time value corresponds to a (more or less) unique and well defined point in time.
The downside, as you've noticed, is that arithmetic using "fuzzy" time units like months or years gets difficult: you can define a year as 365.25 days (or as 365.2425 days, to take into account that only 97 out of every 400 years are leap years in the Gregorian calendar) and a month as 1/12 years, but this will cause adding a year to a date-time value to also shift the time of day by (about) 6 hours, which may be unexpected.
This approach also doesn't let you easily represent "floating" time value, like times of day without a specified date and time zone. (You can sort of deal with floating time zones by doing your time math in UTC and just pretending that it's in your local time zone, but this can cause weird stuff to happen around DST changeovers.) Conversely, it can also cause difficulties if you need to represent imprecise date/time values, such as dates without a time component.
In particular, if you choose the "natural" representation, where imprecise datetimes are represented by their starting point, so that e.g. an unspecified time of day defaults to 00:00:00.0, then anything that causes the time part to be reduced by even a fraction of a second — like, say, shifting to a later time zone, or subtracting a fuzzy time unit that is not an integral number of days — will flip the date part to the previous day. For example, with this representation, subtracting one year (= 265.2425 days) from January 1, 2014 will yield a date in 2012 (specifically, December 31, 2012, 17:56:32)!
You can avoid some of these issues by representing imprecise date/time values by their midpoints instead, so that e.g. the date 2014 is treated as shorthand for June 2, 2014, 12:00:00. What you lose, with this representation, is the ability to build datetimes just by adding up components: with this representation, 2014 + 5 months + 3 days isn't anywhere near May 3, 2014.
Also, just when you think you've at least got simple non-fuzzy time arithmetic unambiguously sorted out, someone's going to tell you about leap seconds...
The alternative approach is to store datetime values in decomposed year / month / day / hour / minute / second / etc. format. With this presentation, time intervals are also naturally stored in a decomposed format: "one month + 17 days" is, in itself, a valid time interval in such a representation, and need not (and should not) be simplified further.
This has a few obvious advantages:
Fuzzy unit arithmetic is (conceptually) simple: to add one year to a date, just increment the year component by one.
Imprecise date/time values can be naturally represented: for a pure date value, the time-of-day components can simply be left undefined (= e.g. represented by negative values for the undefined components, or simply by having each datetime value store its precision).
You have precise control over when and if rollover occurs: adding a year to a date in 2014 will always yield a date in 2015.
You can also support floating time values, such as times of day without a specified date, or dates of year without a specified year. Floating time zones also become supportable.
That said, there are some disadvantages, too:
Implementing date arithmetic gets more complex, since you have to deal with non-trivial carry/borrow rules. (Quick! What's the date 10,000,000 seconds after May 3, 2014?)
You'll still have ambiguities with month arithmetic: what's the date one month after January 31? And does it depend on whether it's a leap year or not?
You can allow such a format to store "impossible" dates like "February 31", with an optional method to normalize them to, say, February 28 (or 29, for a leap year) later. This has the advantage of preserving (some) arithmetic consistency: it allows (January 31 + 1 month) + 1 month to equal March 31 as expected.
In some ways, though this merely postpones the problem: presumably, January 31 + 24 hours should fall on February 1, but what day and month should January 31 + 1 month + 24 hours fall on? The "obvious" choice would be March 1, but whatever you choose, there will be some sequence of arithmetic operations that will yield inconsistent results.