[Haskell-cafe] Data.Time

[Yitzchak Gale]

Joe Healy wrote:
>> One of the points I found "non obvious" were the fact that local time is
>> just that. There is no knowledge of the actual timezone in the data
>> type. If you wish to store that, it needs to be stored alongside.

Erik Hesselink wrote:
> Isn't that what ZonedTime [1] is for?

Not exactly. A "TimeZone" in Data.Time doesn't really
represent a time zone - it represents a specific clock setting
in a time zone. Clock settings periodically change in a time
zone - e.g., to or from summer time, or due to government
decree. A ZonedTime is a LocalTime together with a
specific clock setting in a time zone. There is no guarantee
that the clock setting was actually in effect at the given
LocalTime. So a ZonedTime is only useful if you have checked
separately that it is consistent, or if you don't care.

ZoneSeriesTime in the timezone-series package[1] more
accurately represents a moment in time in a given time
zone. There the moment is represented as a UTCTime,
not a LocalTime. The reason is that a LocalTime in a
time zone does not necessarily represent a unique moment.
When the LocalTime happens to be near a clock change,
it can represent zero moments or more than one moment.

To get the LocalTime from a ZoneSeriesTime, use the
function zoneSeriesTimeToLocalTime.

To get a TimeZoneSeries, representing a time zone with
all of its known clock changes throughout history and some
years into the future, use the timezone-olson package[2] to
read an Olson time zone file. On Linux and Mac OS X
systems, Olson time zone files are available in the directory
/usr/share/zoneinfo.

-Yitz