Ruby – Difference between DateTime and Time in Ruby

datetimerubytime

What's the difference between DateTime and Time classes in Ruby and what factors would cause me to choose one or the other?

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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.

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C# – Calculate relative time in C#

Jeff, your code is nice but could be clearer with constants (as suggested in Code Complete).

const int SECOND = 1;
const int MINUTE = 60 * SECOND;
const int HOUR = 60 * MINUTE;
const int DAY = 24 * HOUR;
const int MONTH = 30 * DAY;

var ts = new TimeSpan(DateTime.UtcNow.Ticks - yourDate.Ticks);
double delta = Math.Abs(ts.TotalSeconds);

if (delta < 1 * MINUTE)
  return ts.Seconds == 1 ? "one second ago" : ts.Seconds + " seconds ago";

if (delta < 2 * MINUTE)
  return "a minute ago";

if (delta < 45 * MINUTE)
  return ts.Minutes + " minutes ago";

if (delta < 90 * MINUTE)
  return "an hour ago";

if (delta < 24 * HOUR)
  return ts.Hours + " hours ago";

if (delta < 48 * HOUR)
  return "yesterday";

if (delta < 30 * DAY)
  return ts.Days + " days ago";

if (delta < 12 * MONTH)
{
  int months = Convert.ToInt32(Math.Floor((double)ts.Days / 30));
  return months <= 1 ? "one month ago" : months + " months ago";
}
else
{
  int years = Convert.ToInt32(Math.Floor((double)ts.Days / 365));
  return years <= 1 ? "one year ago" : years + " years ago";
}

Ruby – the difference between include and require in Ruby

What's the difference between "include" and "require" in Ruby?

Answer:

The include and require methods do very different things.

The require method does what include does in most other programming languages: run another file. It also tracks what you've required in the past and won't require the same file twice. To run another file without this added functionality, you can use the load method.

The include method takes all the methods from another module and includes them into the current module. This is a language-level thing as opposed to a file-level thing as with require. The include method is the primary way to "extend" classes with other modules (usually referred to as mix-ins). For example, if your class defines the method "each", you can include the mixin module Enumerable and it can act as a collection. This can be confusing as the include verb is used very differently in other languages.

Source

So if you just want to use a module, rather than extend it or do a mix-in, then you'll want to use require.

Oddly enough, Ruby's require is analogous to C's include, while Ruby's include is almost nothing like C's include.

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Related Solutions

C# – Calculate relative time in C#

Ruby – the difference between include and require in Ruby

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