Convert day and hour in user time zone - ruby-on-rails-3

Question

I want to convert the group date (day + hour) to user time zone to show the list, i don't want update the db. I have several groups and this code is veeeeery slow.
# GROUP.RB
# day -> Date
# hour -> Time
# wday -> Integer
.
.
groups.each do |g|
new_day = g.in_gmt_day(user_time_zone) # <-- Concat group.day with group.hour and convert the result day with the user time zone.
g.wday = new_day.wday
g.hour = new_day
end
.
.
Any one can help me?
Thanks.

Answer 1

Let Postgres do this for you:
SELECT (day + hour) AT TIME ZONE '<source tz>' AT TIME ZONE 'localtime'
FROM groups;
This does
create a timestamp [without time zone] out of the two columns of data types date and time (assuming here, the question is vague about the exact data types):
day + hour
Set this timestamp at the time zone of the source, let's use 'Europe/London' as example. Results in a timestamp with time zone (or timestamptz for short):
(day + hour) AT TIME ZONE 'Europe/London'
Show the equivalent local timestamp [without time zone]:
(day + hour) AT TIME ZONE 'Europe/London' AT TIME ZONE 'localtime'
From here you can do anything with the value, like split up in day and time again, or format it any way you like with to_char():
WITH x AS (
SELECT (day + hour) AT TIME ZONE 'Europe/London' AT TIME ZONE 'localtime' AS ts
FROM groups
)
SELECT ts::date AS day, ts::time AS hour, to_char(ts, 'HH24') AS h24
FROM x;