From: Tim Pease <tim.pease@...>
Date: 2006-08-09T04:22:40+09:00
Subject: Re: solar local time

On 8/8/06, ara.t.howard@noaa.gov <ara.t.howard@noaa.gov> wrote:
>
> i'm working on a sticky issue:  i'm converting some times based on a utc time
> and longitude - the goal is the express the time as solar local time.
>
> the code i have is close.  it does something like:
>
>
>      DEG_PER_HR = 15.0
>      SECS_PER_MIN = 60.0
>      SECS_PER_HR = SECS_PER_MIN * 60.0
>      SECS_PER_DAY = SECS_PER_HR * 24
>
>      # longitudes > 180 degrees are behind UT
>      # longitudes <= 180 degrees are ahead of UT
>
>      offset_seconds =
>        if longitude > 180.0
>          - ((360.0 - longitude) / DEG_PER_HR) * SECS_PER_HR
>        else
>          (longitude / DEG_PER_HR) * SECS_PER_HR
>        end
>
>      utc + offset_seconds
>
>
> this in fact yields the correct time __except__ that's it's expressed in zulu
> time and not the 'local' time zone.  the trick is that 'local' here means
> 'there' not 'here'!  ;-)  in otherwords it needs to be expressed in the local
> time zone of that __longitude__ not mine and not that of greenwich.
>
> to put it more consisely i need to be able to follow
>
>    locatime_but_in_utc = utc + offset_seconds
>
> with
>
>    move_to_zone locatime_but_in_utc, longitude
>
> or some other viable alternative.
>
> thoughts?
>

ZONES = class << Time; ZoneOffset.keys.map {|x| x.length == 1 ? x :
nil}.compact.sort; end
ZONES.unshift ZONES.pop

def move_to_zone( utc, longitude )
  zone = ZONES[Integer(longitude) / Integer(DEG_PER_HR)]
  utc - Time.zone_offset(zone)
end


This does not take into account daylight savings, etc.  But give it a
shot and see if it's what you need.

TwP