Add Format Date And Time With Time Constants as a Go TIL

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1537 TILs and counting..._
+_1538 TILs and counting..._
 
 ---
 
@@ -405,6 +405,7 @@ _1537 TILs and counting..._
 - [Combine Two Slices](go/combine-two-slices.md)
 - [Do Something N Times](go/do-something-n-times.md)
 - [Find Executables Installed By Go](go/find-executables-installed-by-go.md)
+- [Format Date And Time With Time Constants](go/format-date-and-time-with-time-constants.md)
 - [Not So Random](go/not-so-random.md)
 - [Parse A String Into Individual Fields](go/parse-a-string-into-individual-fields.md)
 - [Parse Flags From CLI Arguments](go/parse-flags-from-cli-arguments.md)

go/format-date-and-time-with-time-constants.md

@@ -0,0 +1,51 @@
+# Format Date And Time With Time Constants
+
+The Go [`time` package](https://pkg.go.dev/time) has a [`Format`
+function](https://pkg.go.dev/time#Time.Format) for displaying the parts of a
+date and time in standard and custom ways. It works a bit different than you
+might be used to from other languages. Rather than using `strftime` identifiers
+like in this string `"%B %d, %Y"`, there is a canonical date that is used as a
+reference point.
+
+That canonical date is from Janary 2nd, 2006. That was a Monday. It was at 5
+seconds after 3:04PM. The Unix format of it looks like `"Mon Jan _2 15:04:05
+MST 2006"`.
+
+```
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+func main() {
+	// This specific time pulled from `time.Format` docs
+	t, _ := time.Parse(time.UnixDate, "Wed Feb 25 11:06:39 PST 2015")
+
+	// Reference date and time:
+	// "Mon Jan _2 15:04:05 MST 2006"
+
+	strf1 := t.Format("|2006|02|01|03:04:05|Day: Mon|")
+	fmt.Println("strf1:", strf1)
+	// strf1: |2015|25|02|11:06:39|Day: Wed|
+
+	strf2 := t.Format(time.DateTime)
+	strf3 := t.Format(time.RubyDate)
+	strf4 := t.Format(time.Kitchen)
+
+	fmt.Println("DateTime:", strf2) // DateTime: 2015-02-25 11:06:39
+	fmt.Println("RubyDate:", strf3) // RubyDate: Wed Feb 25 11:06:39 +0000 2015
+	fmt.Println("Kitchen:", strf4)  // Kitchen: 11:06AM
+}
+```
+
+Though there are a [variety of useful formatting
+constants](https://pkg.go.dev/time#pkg-constants) already available like
+`DateTime`, `RubyDate`, `Kitchen`, etc., we can also define our own formatting
+string by using the reference values for each part of a date and time.
+
+If you want to reference the year, whether as `YYYY` or `YY`, it is always
+going to be a form of `2006`, so `2006` or `06` respectively. Even though the
+above time variable is in February, our format strings will always need to use
+one of `Jan`, `January`, `01` or `1`.