Why does this rails query behave differently depending on timezone?

Question

I have a rails time-based query which has some odd timezone sensitive behaviour, even though as far as I know I'm using UTC. In a nutshell, these queries give different answers:

>> Model.find(:all,:conditions=>['created_at<=?',(Time.now-1.hours).gmtime]).length
=> 279
>> Model.find(:all,:conditions=>['created_at<=?',(Time.now-1.hours)]).length
=> 280

Where the DB actually does contain one model created in the last hour, and the total number of models is 280. So only the first query is correct.

However, in environment.rb I have:

config.time_zone = 'UTC'

The system time zone (as reported by 'date') is BST (which is GMT+1) - so somehow this winds up getting treated as UTC and breaking queries.

This is causing me all sorts of problems as I need to parameterise the query passing in different times to an action (which are then converted using Time.parse()), and even though I send in UTC times, this 'off by one hour' DST issue crops a lot. Even using '.gmtime()' doesn't always seem to fix it.

Obviously the difference is caused somehow by an implicit conversion somewhere resulting in BST being incorrectly treated as UTC, but why? Doesn't rails store the timestamps in UTC? Isn't the Time class timezone aware? I am using Rails 2.2.2

So what is going on here - and what is the safe way to program around it?

edit, some additional info to show what the DB and Time class are doing:

>> Model.find(:last).created_at
=> Tue, 11 Aug 2009 20:31:07 UTC +00:00
>> Time.now
=> Tue Aug 11 22:00:18 +0100 2009
>> Time.now.gmtime
=> Tue Aug 11 21:00:22 UTC 2009

Answer 1

The Time class isn't directly aware of your configured timezone. Rails 2.1 added a bunch of timezone support, but Time will still act upon your local timezone. This is why Time.now returns a BST time.

What you likely want is to interact with Time.zone. You can call methods on this like you would the Time class itself but it will return it in the specified time zone.

Time.zone.now # => Tue, 11 Aug 2009 21:31:45 UTC +00:00
Time.zone.parse("2:30 PM Aug 23, 2009") # => Sun, 23 Aug 2009 14:30:00 UTC +00:00

Another thing you have to be careful with is if you ever do queries on the database where you are comparing times, but sure to use the UTC time (even if you have a different time zone specified) because Rails always stores UTC in the database.

Item.all(:conditions => ["published_at <= ?", Time.now.utc])

Also, instead of Time.now-1.hour do 1.hour.ago. It is easier to read and Rails will automatically use the configured timezone.