Add Format DateTime With Builtin Formats as a Rails TIL

rails/format-datetime-with-builtin-formats.md

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+# Format DateTime With Builtin Formats
+
+The Rails [`Date`](https://api.rubyonrails.org/classes/Date.html)/`DateTime`
+and [`Time`](https://api.rubyonrails.org/classes/Time.html) classes each come
+with a `DATE_FORMATS` constant that is a hash of symbol names to format
+strings.
+
+```ruby
+> DateTime::DATE_FORMATS
+=>
+{:short=>"%d %b",
+ :long=>"%B %d, %Y",
+ :db=>"%Y-%m-%d",
+ :inspect=>"%Y-%m-%d",
+ :number=>"%Y%m%d",
+ :long_ordinal=>
+  #<Proc:0x0000000105b2cef0 /Users/jbranchaud/.local/share/mise/installs/ruby/3.2.2/lib/ruby/gems/3.2.0/gems/activesupport-8.0.1/lib/active_support/core_ext/date/conversions.rb:15 (lambda)>,
+ :rfc822=>"%d %b %Y",
+ :rfc2822=>"%d %b %Y",
+ :iso8601=>
+  #<Proc:0x0000000105b2cec8 /Users/jbranchaud/.local/share/mise/installs/ruby/3.2.2/lib/ruby/gems/3.2.0/gems/activesupport-8.0.1/lib/active_support/core_ext/date/conversions.rb:21 (lambda)>}
+```
+
+These can be used as a standardized, ready-to-use, named formats when turning
+`DateTime` objects into strings.
+
+Here are a few examples
+
+```ruby
+> now = DateTime.now
+=> Wed, 30 Apr 2025 23:08:08 -0500
+
+> now.to_fs(:long)
+=> "April 30, 2025 23:08"
+
+> now.to_fs(:long_ordinal)
+=> "April 30th, 2025 23:08"
+
+> now.to_fs(:iso8601)
+=> "2025-04-30T23:08:08-05:00"
+```
+
+If an unrecognized key is passed to `#to_fs`, then it falls back to the
+`:iso8601` format.
+
+```ruby
+> now.to_fs(:taco_bell)
+=> "2025-04-30T23:08:08-05:00"
+```