Question

I create multiple scheduled objects with different scheduled_on attributes. For example, each object would have a date to land on 4:00pm the first of every month.

Once one of those objects hits a timezone change. The app intelligently configures it an hour ahead or behind so that its relative to its parent’s timezone.

The problem is that the app will save an object as 4:00PM (in Pacific Standard) for times that will eventually be displayed as (PDT or an hour ahead or 5:00pm). This would mean that I need it to save an hour off in UTC so that when the time comes about, it will display as 4PM regardless of what timezone we are in.

Whats the best technique for ensuring this in Rails?

Answer 1

I’m going to answer this question by pointing out some good things to know about adding time in Rails in relation to timezone.

When you add time, time is allocated in UTC to stay the same time despite timezone changes :

t = Time.now
-> 2012-08-10 13:17:01 +0200
t + 90.days
-> 2012-11-08 13:17:01 +0100

A DateTime will not do this. A DateTime will go up an hour or down an hour in the same TimeZone it began in :

 dt = DateTime.now
 => Fri, 10 Aug 2012 13:16:54 +0200 
 dt + 90.days
 => Thu, 08 Nov 2012 13:16:54 +0200

But a DateTime is the only way to get the number of days between two dates which you can do by subtracting two DateTimes. This, you can’t do with a Time, because when substracting a time, it will not divide by a perfect 24 hours, you’ll get an irrational number because of the timezone switch.

This is specific to my issue. But I solved my problem by converting my Time to DateTimes to find the number of days in distance, and then reconverted back to time to find a later time in UTC relative to a TimeZone change :

original_job.to_time + ( new_date.to_datetime - original_job.to_datetime ).to_i.days