Difference between DateTime and Time in Ruby

Newer versions of Ruby (2.0+) do not really have significant differences between the two classes. Some libraries will use one or the other for historical reasons, but new code does not necessarily need to be concerned. Picking one for consistency is probably best, so try and mesh with what your libraries expect. For example, ActiveRecord prefers DateTime.

In versions prior to Ruby 1.9 and on many systems Time is represented as a 32-bit signed value describing the number of seconds since January 1, 1970 UTC, a thin wrapper around a POSIX-standard time_t value, and is bounded:

Time.at(0x7FFFFFFF)
# => Mon Jan 18 22:14:07 -0500 2038
Time.at(-0x7FFFFFFF)
# => Fri Dec 13 15:45:53 -0500 1901

Newer versions of Ruby are able to handle larger values without producing errors.

DateTime is a calendar-based approach where the year, month, day, hour, minute and second are stored individually. This is a Ruby on Rails construct that serves as a wrapper around SQL-standard DATETIME fields. These contain arbitrary dates and can represent nearly any point in time as the range of expression is typically very large.

DateTime.new
# => Mon, 01 Jan -4712 00:00:00 +0000

So it's reassuring that DateTime can handle blog posts from Aristotle.

When choosing one, the differences are somewhat subjective now. Historically DateTime has provided better options for manipulating it in a calendar fashion, but many of these methods have been ported over to Time as well, at least within the Rails environment.

I think the answer to "what's the difference" is one of the unfortunate common answers to this question in the Ruby standard libraries: the two classes/libs were created differently by different people at different times. It's one of the unfortunate consequences of the community nature of Ruby's evolution compared to carefully planned development of something like Java. Developers want new functionality but don't want to step on existing APIs so they just create a new class - to the end user there's no obvious reason for the two to exist.

This is true for software libraries in general: often the reason some code or API is the way it is turns out to be historical rather than logical.

The temptation is to start with DateTime because it seems more generic. Date... and Time, right? Wrong. Time also does dates better, and in fact can parse timezones where DateTime can't. Also it performs better.

I've ended up using Time everywhere.

To be safe though, I tend to allow for DateTime arguments to be passed into my Timey APIs, and either convert. Also if I know that both have the method I'm interested in I accept either, like this method I wrote for converting times to XML (for XMLTV files)

# Will take a date time as a string or as a Time or DateTime object and
# format it appropriately for xmtlv. 
# For example, the 22nd of August, 2006 at 20 past midnight in the British Summertime
# timezone (i.e. GMT plus one hour for DST) gives: "20060822002000 +0100"
def self.format_date_time(date_time)
  if (date_time.respond_to?(:rfc822)) then
    return format_time(date_time)
  else 
    time = Time.parse(date_time.to_s)
    return format_time(time)
  end    
end

# Note must use a Time, not a String, nor a DateTime, nor Date.
# see format_date_time for the more general version
def self.format_time(time)
  # The timezone feature of DateTime doesn't work with parsed times for some reason
  # and the timezone of Time is verbose like "GMT Daylight Saving Time", so the only
  # way I've discovered of getting the timezone in the form "+0100" is to use 
  # Time.rfc822 and look at the last five chars
  return "#{time.strftime( '%Y%m%d%H%M%S' )} #{time.rfc822[-5..-1]}"
end

In addition to the answer of Niels Ganser you might consider this argument:

Note that The Ruby Style Guide quite clearly states a position on this:

No DateTime

Don’t use DateTime unless you need to account for historical calendar reform - and if you do, explicitly specify the start argument to clearly state your intentions.

# bad - uses DateTime for current time
DateTime.now

# good - uses Time for current time
Time.now

# bad - uses DateTime for modern date
DateTime.iso8601('2016-06-29')

# good - uses Date for modern date
Date.iso8601('2016-06-29')

# good - uses DateTime with start argument for historical date
DateTime.iso8601('1751-04-23', Date::ENGLAND)

It seems that in some cases the behavior is very different:

Time.parse("Ends from 28 Jun 2018 12:00 BST").utc.to_s

"2018-06-28 09:00:00 UTC"

Date.parse("Ends from 28 Jun 2018 12:00 BST").to_time.utc.to_s

"2018-06-27 21:00:00 UTC"

DateTime.parse("Ends from 28 Jun 2018 12:00 BST").to_time.utc.to_s

"2018-06-28 11:00:00 UTC"