Times and Dates
Time
Time is an abstraction of dates and times. Time is stored internally as the number of seconds with fraction since the Epoch, January 1, 1970 00:00 UTC. Also see the library module Date. The Time class treats GMT (Greenwich Mean Time) and UTC (Coordinated Universal Time) as equivalent. GMT is the older way of referring to these baseline times but persists in the names of calls on POSIX systems.
All times may have fraction. Be aware of this fact when comparing times with each other – times that are apparently equal when displayed may be different when compared.
Since Ruby 1.9.2, Time implementation uses a signed 63 bit integer, Bignum or Rational. The integer is a number of nanoseconds since the Epoch which can represent 1823-11-12 to 2116-02-20. When Bignum or Rational is used (before 1823, after 2116, under nanosecond), Time works slower as when integer is used.
Creating a new Time instance
You can create a new instance of Time with Time::new. This will use the current system time. Time::now is an alias for this. You can also pass parts of the time to Time::new such as year, month, minute, etc. When you want to construct a time this way you must pass at least a year. If you pass the year with nothing else time will default to January 1 of that year at 00:00:00 with the current system timezone. Here are some examples:
Time.new(2002) #=> 2002-01-01 00:00:00 -0500
Time.new(2002, 10) #=> 2002-10-01 00:00:00 -0500
Time.new(2002, 10, 31) #=> 2002-10-31 00:00:00 -0500
You can pass a UTC offset:
Time.new(2002, 10, 31, 2, 2, 2, "+02:00") #=> 2002-10-31 02:02:02 +0200
Or a timezone object:
tz = timezone("Europe/Athens") # Eastern European Time, UTC+2
Time.new(2002, 10, 31, 2, 2, 2, tz) #=> 2002-10-31 02:02:02 +0200
You can also use Time::gm, Time::local and Time::utc to infer GMT, local and UTC timezones instead of using the current system setting.
You can also create a new time using Time::at which takes the number of seconds (or fraction of seconds) since the Unix Epoch.
Time.at(628232400) #=> 1989-11-28 00:00:00 -0500
Working with an instance of Time
Once you have an instance of Time there is a multitude of things you can do with it. Below are some examples. For all of the following examples, we will work on the assumption that you have done the following:
t = Time.new(1993, 02, 24, 12, 0, 0, "+09:00")
Was that a monday?
t.monday? #=> false
What year was that again?
t.year #=> 1993
Was it daylight savings at the time?
t.dst? #=> false
What’s the day a year later?
t + (60*60*24*365) #=> 1994-02-24 12:00:00 +0900
How many seconds was that since the Unix Epoch?
t.to_i #=> 730522800
You can also do standard functions like compare two times.
t1 = Time.new(2010)
t2 = Time.new(2011)
t1 == t2 #=> false
t1 == t1 #=> true
t1 < t2 #=> true
t1 > t2 #=> false
Time.new(2010,10,31).between?(t1, t2) #=> true
Timezone argument
A timezone argument must have local_to_utc and utc_to_local methods, and may have name, abbr, and dst? methods.
The local_to_utc method should convert a Time-like object from the timezone to UTC, and utc_to_local is the opposite. The result also should be a Time or Time-like object (not necessary to be the same class). The #zone of the result is just ignored. Time-like argument to these methods is similar to a Time object in UTC without sub-second; it has attribute readers for the parts, e.g. #year, #month, and so on, and epoch time readers, #to_i. The sub-second attributes are fixed as 0, and #utc_offset, #zone, #isdst, and their aliases are same as a Time object in UTC. Also #to_time, #+, and #- methods are defined.
The name method is used for marshaling. If this method is not defined on a timezone object, Time objects using that timezone object can not be dumped by Marshal.
The abbr method is used by ‘%Z’ in #strftime.
The dst? method is called with a Time value and should return whether the Time value is in daylight savings time in the zone.
Auto conversion to Timezone
At loading marshaled data, a timezone name will be converted to a timezone object by find_timezone class method, if the method is defined.
Similarly, that class method will be called when a timezone argument does not have the necessary methods mentioned above.
time.rb
Part of the useful functionality for Time is provided by the standard library time.
Examples:
require 'time'
t = Time.now
t.iso8601 # => "2011-10-05T22:26:12-04:00"
t.rfc2822 # => "Wed, 05 Oct 2011 22:26:12 -0400"
t.httpdate # => "Thu, 06 Oct 2011 02:26:12 GMT"
Time.parse("2010-10-31") #=> 2010-10-31 00:00:00 -0500
Time.strptime("2000-10-31", "%Y-%m-%d") #=> 2000-10-31 00:00:00 -0500
Date
Part of standard library. You need to require 'date' before using.
A subclass of Object that includes the Comparable module and easily handles date.
A Date object is created with Date::new, Date::jd, Date::ordinal, Date::commercial, Date::parse, Date::strptime, Date::today, Time#to_date, etc.
require 'date'
Date.new(2001,2,3)
#=> #<Date: 2001-02-03 ...>
Date.jd(2451944)
#=> #<Date: 2001-02-03 ...>
Date.ordinal(2001,34)
#=> #<Date: 2001-02-03 ...>
Date.commercial(2001,5,6)
#=> #<Date: 2001-02-03 ...>
Date.parse('2001-02-03')
#=> #<Date: 2001-02-03 ...>
Date.strptime('03-02-2001', '%d-%m-%Y')
#=> #<Date: 2001-02-03 ...>
Time.new(2001,2,3).to_date
#=> #<Date: 2001-02-03 ...>
All date objects are immutable; hence cannot modify themselves.
The concept of a date object can be represented as a tuple of the day count, the offset and the day of calendar reform.
The day count denotes the absolute position of a temporal dimension. The offset is relative adjustment, which determines decoded local time with the day count. The day of calendar reform denotes the start day of the new style. The old style of the West is the Julian calendar which was adopted by Caesar. The new style is the Gregorian calendar, which is the current civil calendar of many countries.
The day count is virtually the astronomical Julian day number. The offset in this class is usually zero, and cannot be specified directly.
A Date object can be created with an optional argument, the day of calendar reform as a Julian day number, which should be 2298874 to 2426355 or negative/positive infinity. The default value is Date::ITALY (2299161=1582-10-15). See also sample/cal.rb.
$ ruby sample/cal.rb -c it 10 1582
October 1582
S M Tu W Th F S
1 2 3 4 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
$ ruby sample/cal.rb -c gb 9 1752
September 1752
S M Tu W Th F S
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
A Date object has various methods. See each reference.
d = Date.parse('3rd Feb 2001')
#=> #<Date: 2001-02-03 ...>
d.year #=> 2001
d.mon #=> 2
d.mday #=> 3
d.wday #=> 6
d += 1 #=> #<Date: 2001-02-04 ...>
d.strftime('%a %d %b %Y') #=> "Sun 04 Feb 2001"
DateTime
Part of standard library. You need to require 'date' before using.
A subclass of Date that easily handles date, hour, minute, second, and offset.
DateTime does not consider any leap seconds, does not track any summer time rules.
A DateTime object is created with DateTime::new, DateTime::jd, DateTime::ordinal, DateTime::commercial, DateTime::parse, DateTime::strptime, DateTime::now, Time#to_datetime, etc.
require 'date'
DateTime.new(2001,2,3,4,5,6)
#=> #<DateTime: 2001-02-03T04:05:06+00:00 ...>
The last element of day, hour, minute, or second can be a fractional number. The fractional number’s precision is assumed at most nanosecond.
DateTime.new(2001,2,3.5)
#=> #<DateTime: 2001-02-03T12:00:00+00:00 ...>
An optional argument, the offset, indicates the difference between the local time and UTC. For example, Rational(3,24) represents ahead of 3 hours of UTC, Rational(-5,24) represents behind of 5 hours of UTC. The offset should be -1 to +1, and its precision is assumed at most second. The default value is zero (equals to UTC).
DateTime.new(2001,2,3,4,5,6,Rational(3,24))
#=> #<DateTime: 2001-02-03T04:05:06+03:00 ...>
The offset also accepts string form:
DateTime.new(2001,2,3,4,5,6,'+03:00')
#=> #<DateTime: 2001-02-03T04:05:06+03:00 ...>
An optional argument, the day of calendar reform (start), denotes a Julian day number, which should be 2298874 to 2426355 or negative/positive infinity. The default value is Date::ITALY (2299161=1582-10-15).
A DateTime object has various methods. See each reference.
d = DateTime.parse('3rd Feb 2001 04:05:06+03:30')
#=> #<DateTime: 2001-02-03T04:05:06+03:30 ...>
d.hour #=> 4
d.min #=> 5
d.sec #=> 6
d.offset #=> (7/48)
d.zone #=> "+03:30"
d += Rational('1.5')
#=> #<DateTime: 2001-02-04%16:05:06+03:30 ...>
d = d.new_offset('+09:00')
#=> #<DateTime: 2001-02-04%21:35:06+09:00 ...>
d.strftime('%I:%M:%S %p')
#=> "09:35:06 PM"
d > DateTime.new(1999)
#=> true
When should you use DateTime and when should you use Time?
It’s a common misconception that William Shakespeare and Miguel de Cervantes died on the same day in history - so much so that UNESCO named April 23 as World Book Day because of this fact. However, because England hadn’t yet adopted the Gregorian Calendar Reform (and wouldn’t until 1752) their deaths are actually 10 days apart. Since Ruby’s Time class implements a proleptic Gregorian calendar and has no concept of calendar reform there’s no way to express this with Time objects. This is where DateTime steps in:
shakespeare = DateTime.iso8601('1616-04-23', Date::ENGLAND)
#=> Tue, 23 Apr 1616 00:00:00 +0000
cervantes = DateTime.iso8601('1616-04-23', Date::ITALY)
#=> Sat, 23 Apr 1616 00:00:00 +0000
Already you can see something is weird - the days of the week are different. Taking this further:
cervantes == shakespeare
#=> false
(shakespeare - cervantes).to_i
#=> 10
This shows that in fact they died 10 days apart (in reality 11 days since Cervantes died a day earlier but was buried on the 23rd). We can see the actual date of Shakespeare’s death by using the #gregorian method to convert it:
shakespeare.gregorian
#=> Tue, 03 May 1616 00:00:00 +0000
So there’s an argument that all the celebrations that take place on the 23rd April in Stratford-upon-Avon are actually the wrong date since England is now using the Gregorian calendar. You can see why when we transition across the reform date boundary:
# start off with the anniversary of Shakespeare's birth in 1751
shakespeare = DateTime.iso8601('1751-04-23', Date::ENGLAND)
#=> Tue, 23 Apr 1751 00:00:00 +0000
# add 366 days since 1752 is a leap year and April 23 is after February 29
shakespeare + 366
#=> Thu, 23 Apr 1752 00:00:00 +0000
# add another 365 days to take us to the anniversary in 1753
shakespeare + 366 + 365
#=> Fri, 04 May 1753 00:00:00 +0000
As you can see, if we’re accurately tracking the number of solar years since Shakespeare’s birthday then the correct anniversary date would be the 4th May and not the 23rd April.
So when should you use DateTime in Ruby and when should you use Time? Almost certainly you’ll want to use Time since your app is probably dealing with current dates and times. However, if you need to deal with dates and times in a historical context you’ll want to use DateTime to avoid making the same mistakes as UNESCO. If you also have to deal with timezones then best of luck - just bear in mind that you’ll probably be dealing with local solar times, since it wasn’t until the 19th century that the introduction of the railways necessitated the need for Standard Time and eventually timezones.