Handling of days and weeks in CalendarDiffTime

[dcastro]

If I understand things right, as of #40, we can parse "P1DT1S" to CalendarDiffTime using durationTimeFormat:

However, CalendarDiffTime normalizes the "days" and "weeks" units (D and W) and assumes all days have a standard 24 hours.

> formatParseM durationTimeFormat "P1DT1S"
P0MT86401S

On the other hand, the parser durationDaysFormat for CalendarDiffDays does correctly handle "days" and "weeks", but it does not handle smaller time units like hours and seconds.

Should there be a type that handles strings like P1DT1S correctly? A type with 3 fields, for months, days, and NominalDiffTime?

[AshleyYakeley]

Hmm, it would be better if the CalendarDiffTime type were changed...

[dcastro]

Yes I thought so too, but didn't want to suggest a breaking change ^^
If you're okay with that, I'm all for it!

[AshleyYakeley]

So this part I definitely want to fix:

> formatParseM durationTimeFormat "P1DT1S"
P0MT86401S

The Show instance should use larger units, and in this case should return "P1DT1S".

On the other hand, I would prefer 12H + 12H = 1D: this is what people expect. I don't think it's worth supporting leap-seconds in this type. So I don't think the type should be changed.

[AshleyYakeley]

OK, fixed the Read and Show instances to use the ISO8601 code.