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

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

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+# 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`.