How to convert Date into UTC in MongoMapper & Ruby/Rails?

I added this line of code

self.auth_histo开发者_开发问答ry.push [start_date, self.coupon_code]

And got this error message

Date is not currently supported; use a UTC Time instance instead.

I also tried start_date.utc, but it didn't work either.

Please help. Thanks.

I got this answer from Seattle Brigade group -

===

I didn't see start_date defined in your code as a key in MongoMapper, so I'll assume you're creating your own date object, either directly via Ruby, or wrapped by Rails. As far as I know, and someone please correct me, Mongo stores dates as UTC time in milliseconds since epoch. So when you define a key with a :date mapping in MongoMapper, you're wrapping a Time object in Ruby.

Therefore, if you want to store a date inside of Mongo, and it wasn't created by MongoMapper, make sure you create a Time object in UTC. MongoMapper comes with a Date mixin method called to_mongo that you can use.

>> Time.now.utc
=> Fri Jan 28 03:47:50 UTC 2011
>> require 'date'
=> true
>> date = Date.today
=> #<Date: 4911179/2,0,2299161>
>> Time.utc(date.year, date.month, date.day)
=> Thu Jan 27 00:00:00 UTC 2011
>> require 'rubygems'
=> true
>> require 'mongo_mapper'
=> true
>> Date.to_mongo(date)
=> Thu Jan 27 00:00:00 UTC 2011

But watch out for the time change.

>> Date.to_mongo(Time.now)
=> Thu Jan 27 00:00:00 UTC 2011
>> Date.to_mongo(Time.now.utc)
=> Fri Jan 28 00:00:00 UTC 2011

Good luck.

===

And by using

Date.to_mongo(start_date) 

it works for me.

First, I think the question title is bad in description. Actually, the difference between different timezone is on Time not on Date. So, it's really not proper to say I want to convert a date to UTC format.

Here is another way in Ruby to convert DateTime to its UTC format:

DateTime.now.new_offset(0)

Here's another option:

Time.at(Date.today.to_datetime.to_i).utc

Here I am using Date.today as an arbitrary example date. Replace with whatever date you want to convert. Once the date is converted to a Time instance, it can be serialized to BSON without any problem, as Time is a supported primitive type, that is to say, it can be saved using MongoMapper to the database.

As per EfratBlaier's comment I have updated the answer.

Date.today.to_time.utc