# When should you use DateTime and when should you use Time?

It's a common misconception that [William Shakespeare][1] and [Miguel de Cervantes][2] died on the same day in history - so much so that UNESCO named April 23 as [World Book Day because of this fact][3]. However because England hadn't yet adopted [Gregorian Calendar Reform][4] (and wouldn't until [1752][5]) their deaths are actually 10 days apart. Since Ruby's `Time` class implements a [proleptic Gregorian calendar][6] and has no concept of calendar reform then there's no way to express this. This is where `DateTime` steps in:

``` irb
>> 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's weird - the days of the week are different, taking this further:

``` irb
>> 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:

``` irb
>> 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:

``` irb
# 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][9] since Shakespeare's birthday then the correct anniversary date would be the 4th May and not the 23rd April.

So when should use 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 and it has support for timezones (system/local and utc), whereas `DateTime` just has offsets from UTC. 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][7], since it wasn't until the 19th century that the introduction of the railways necessitated the need for [Standard Time][8] and eventually timezones.

[1]: http://en.wikipedia.org/wiki/William_Shakespeare
[2]: http://en.wikipedia.org/wiki/Miguel_de_Cervantes
[3]: http://en.wikipedia.org/wiki/World_Book_Day
[4]: http://en.wikipedia.org/wiki/Gregorian_calendar#Gregorian_reform
[5]: http://en.wikipedia.org/wiki/Calendar_(New_Style)_Act_1750
[6]: http://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar
[7]: http://en.wikipedia.org/wiki/Solar_time
[8]: http://en.wikipedia.org/wiki/Standard_time#Great_Britain
[9]: http://en.wikipedia.org/wiki/Tropical_year