# I borrowed heavily from http://blog.shupp.org/2012/08/12/local-timezone-lookups-by-coordinates-with-postgis/ to make # this gist which is geared towards mac users like myself.
# Step 1: Install Postgresql & Postgis + dependencies using brew.  

# Note: I wanted an older version of postgresql (9.1.4) and postgis (1.5.3) so i had to run 'brew versions postgresql' and 'brew versions postgis', then cd to /usr/local/Library/Formula and run the approperiate git commands like:

# postgis 1.5.3
git checkout e9799d5 /usr/local/Library/Formula/postgis.rb
# postgresql 9.1.4
git checkout ed92469 /usr/local/Library/Formula/postgresql.rb

# or to get the latest repositories 'brew update' i think.  Now, time to install everything with one easy command!

brew install postgis

# Step 2: follow the instructions displayed here

brew info postgresql

mkdir -p ~/Library/LaunchAgents
  cp /usr/local/Cellar/postgresql/9.1.4/homebrew.mxcl.postgresql.plist ~/Library/LaunchAgents/
  launchctl load -w ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist

# Step 3: Create database user and database (alternatively you can make a template as shown here https://gist.github.com/1198957)

createuser user_name - n y n
createdb -Ouser_name -Eutf8 app_development
createdb -Ouser_name -Eutf8 app_test

# Step 4: Enable database for spatial awareness
psql -d app_development -f /usr/local/Cellar/postgis/1.5.3/share/postgis/postgis.sql
psql -d app_development -f /usr/local/Cellar/postgis/1.5.3/share/postgis/spatial_ref_sys.sql

# Test Step 4 worked with this command
psql -d app_development -c "SELECT postgis_full_version();"

# output: POSTGIS="1.5.3" GEOS="3.3.5-CAPI-1.7.5" PROJ="Rel. 4.8.0, 6 March 2012" LIBXML="2.7.3" USE_STATS

# Step 5: Download shape files from efele.net.  Download tz_world.zip (28,000 rows), unzip into world folder and look 
# for the tz_world.shp file.  Now use ship2pgsql to create a psql dump file for the tz_world table needed for timezone 
# lookkup by polylines and import it into the database.  I moved my world folder to /usr/local/var/world
# Create the dump.sql file like so (Make sure you're in the "world" directory when you run this.)
/usr/local/Cellar/postgis/1.5.3/bin/shp2pgsql -D tz_world.shp > dump.sql

# Import the dump.sql file
psql -d app_development -f dump.sql

# Test timezone lookup by lat/lon, but first change the table owner to the user for rails
psql -d app_development
ALTER TABLE tz_world OWNER TO user_name;
\q
psql -d app_development -U user_name
SELECT tzid FROM tz_world WHERE ST_Contains(the_geom, ST_MakePoint(-122.420706, 37.776685));

and you should get 

  tzid         
---------------------
 America/Los_Angeles