Calendar Types

[Bayley, Alistair]

> From: Ashley Yakeley [mailto:ashley at semantic.org] 
> 
> * Should we include a timezone field in CalendarTime?

Yes. If a CalendarTime doesn't carry timezone information with it, then the
user must bundle it. They will want to do this because (1) they will be
probably dealing with local time (2) a CalendarTime sans timezone is
equivalent to UTC, and is therefore not much use (i.e. we may as well use
UTC instead).


> * TimeZone represents a fixed offset to UTC. What should it look like 
> internally, and what functions on it should we provide?

There are timezones which are not integral hour offsets, so an (hours,
minutes) pair, or maybe just ticks is sufficient. Or maybe a Duration (see
below). Ticks is probably best from an implementation point-of-view, right?

Should it include the timezone name? Although the name determines the
offset, a given offset could match many names, so to preserve information
we'd probably want to retain the timezone name internally. Do we want to
support timezone names?

Do we want to embed summer-time (daylight-savings) timezone behaviour into
the library? This would obviously be useful for displaying the correct local
time, but makes the timezone part of the library quite complex.


> OK, so here are the basic functions of System.Time.Calendar:
> 
>   utcToCalendar :: TimeZone -> UTCTime -> CalendarTime
> 
>   calendarToUTC :: TimeZone -> CalendarTime -> UTCTime
> 
> CalendarTime should be a "struct", i.e. a datatype with its 
> constructor and access functions exported.


I'm assuming System.Time.Calendar is Gregorian. How about
 - a Duration type
 - some calendar arithmetic functions
 - arithmetic on Durations

e.g.

  data Duration = Duration
  {
    -- this looks familiar...
    durYear    :: Int,
    durMonth   :: Int,
    durDay     :: Int,
    durHour    :: Int,
    durMin     :: Int,
    durSec     :: Int,
    durPicosec :: Integer
  } deriving ...


  calendarAdd :: CalendarTime -> Duration -> CalendarTime
  calendarDiff :: CalendarTime -> CalendarTime -> Duration
  durationAdd :: Duration -> Duration -> Duration
  durationDiff :: Duration -> Duration -> Duration


Alistair.

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