Are the Date, Time, and DateTime classes necessary?

Question

What is the purpose of having Date and Time classes when there is a DateTime class that can handle both?

Answer 1

DateTime is a subclass of Date, so whatever you can do with Date can be done with DateTime. But as tadman and steenslag point out, DateTime is slower. See steenslag's answer for how much slower it is.

With respect to DateTime vs, Time, I found something here.

Citation

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Time is a wrapper around Unix-Epoch.
Date (and DateTime) use rational and a "day zero" for storage. So Time 
is faster but the upper and lower bounds are tied to epoch time (which 
for 32bit epoch times is something around 1970-2040 ... while Date (and DateTime) have an 
almost infinite range but are terribly slow.

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In short, DateTime is an all around superstar, and should be preferred in general, but if you want to optimize to the last bit, using Time can improve performance.

Answer 2

To summarize what the common ruby time classes are:

Time

This is the basic workhorse core ruby time class.

• Has date and time attributes (year, month, day, hour, min, sec, subsec)
• Based on floating-point second intervals from unix epoch (1970-01-01)
• Can handle negative times before unix epoch
• Can handle time arithmetic in units of seconds
• Natively works in either UTC or "local" (system time zone)

There are really 3 kinds of Time object when it comes to dealing with time zones, let's look at a summer time to show DST:

utc = Time.utc(2012,6,1) # => 2012-12-21 00:00:00 UTC
utc.zone       # => "UTC"
utc.dst?       # => false
utc.utc?       # => true
utc.utc_offset # => 0

local = Time.local(2012,6,1) # => 2012-06-01 00:00:00 -0700
local.zone       # => "PDT"
local.dst?       # => true
local.utc?       # => false
local.utc_offset # => -25200

nonlocal = Time.new(2012,6,1,0,0,0, "-07:00") # => 2012-06-01 00:00:00 -0700
nonlocal.zone       # => nil
nonlocal.dst?       # => false
nonlocal.utc?       # => false
nonlocal.utc_offset # => -25200

The last 2 look similar, but beware: you should not do arithmetic with a non-local Time. This is simply a time with a UTC offset and no zone, so it doesn't know the rules of DST. Adding time over the DST boundary will not change the offset and the resulting time-of-day will be wrong.

ActiveSupport::TimeWithZone

This one is worth mentioning here since it's what you use in Rails. Same as Time, plus:

• Can handle any time zone
• Respects DST
• Can convert times between zones

I generally always reach for this when ActiveSupport is available as it takes care of all the time zone pitfalls.

Date

• Has date attributes only (year, month, day)
• Based on integer whole-day intervals from an arbitrary "day zero" (-4712-01-01)
• Can handle date arithmetic in units of whole days
• Can convert between dates in the ancient Julian calendar to modern Gregorian

Date is more useful than Time whenever you deal in whole days: no time zones to worry about! (I'm surprised this doesn't deal with the modern Persian calendar since it knows about the obsolete Julian calendar from centuries ago.)

DateTime

• Has date and time attributes (year, month, day, hour, min, sec)
• Based on fractions of whole-day intervals from an arbitrary "day zero" (-4712-01-01)
• Can handle date arithmetic in units of whole days or fractions

Personally, I never have reason to use this: it's slow, it handles time without considering time zones, and it has an inconsistent interface. I find it leads to confusion whenever you assume you have a Time-like object, but it actually behaves like a Date instead:

Time.new(2012, 12, 31, 0, 0, 0) + 1 == Time.new(2012, 12, 31, 0, 0, 1)
DateTime.new(2012, 12, 31, 0, 0, 0) + 1 == DateTime.new(2013, 1, 1, 0, 0, 0)

Further, it has a meaningless "zone" attribute (note how non-local Time objects warn you that zone == nil), and you can't know anything else about it before turning it into a Time first:

dt = DateTime.new(2012,12,6, 1, 0, 0, "-07:00")
dt.zone # => "-07:00"
dt.utc? # => NoMethodError: undefined method `utc?'
dt.dst? # => NoMethodError: undefined method `dst?'
dt.utc_offset # => NoMethodError: undefined method `utc_offset'

Dealing with microseconds to check for rounding is also a little strange. You would think that because it doesn't have a usec attribute that it only deals in whole numbers, but you'd be wrong:

DateTime.now.usec # => NoMethodError: undefined method `usec'
DateTime.now.to_time.usec => 629399

In short, unless you're dealing with astronomical events in the ancient past and need to convert the Julian date (with time of day) to a modern calendar, please don't use DateTime. If anyone has an actual use case for this class, I'd love to read your comments.

Answer 3

I know there is an accepted answer but I have something to add. The Date class is a heavyweight, academic strength class. It can handle all sorts of RFC's, parse the strangest things and converts julian dates from thousand years ago to gregorian with the reform date of choice. The Time class is lightweight and it does not know of any of this stuff. It's cheaper and that shows up in a benchmark:

require 'benchmark'
require 'date'

Benchmark.bm(10) do |x|
  x.report('date'){100000.times{Date.today} }
  x.report('datetime'){100000.times{DateTime.now} }
  x.report('time'){100000.times{Time.now} }
end

Result:

                user     system      total        real
date        1.250000   0.270000   1.520000 (  1.799531)
datetime    6.660000   0.360000   7.020000 (  7.690016)
time        0.140000   0.030000   0.170000 (  0.200738)

(Ruby 1.9.2)

Answer 4

Yes. Date handles only the date for something, I.E., March 31, 1989. But it does not handle Time, for example, 12:30 PM. DateTime, can handle both, March 31, 1989 12:30 PM EST.

Sometimes you don't need all parts of the DateTime. For example, you wanted to know when the use signed up for you website, Date would be useful here, because the time is eventually irrelevant.

In some cases you might want just the time. For example, if it's lunch time, you may want to tell the user your office is closed. At this point, the Data is irrelevant.

However, in most cases DateTime is used, because it can be used as either date, time, or both.

Answer 5

Another way of thinking of this is that Date and DateTime model time in terms of clocks and calendars, which is useful for describing times to users, and scheduling events. Having a Date without a time is nice for when you don't care about the time, and you don't want to think about time zones.

Time models time as a continuum, and is a wrapper around the Unix timestamp, which is just an integer. This is useful for all manner of internal applications where the computer doesn't care much whether a calendar boundary has been crossed, but just how many seconds (or milliseconds) have elapsed.