Magic Date in Golang: 2006-01-02 15:04:05

2019-08-20 1225 words 6 minutes

Contents

Let’s take a look at the following code:

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package main

import (
    "fmt"
    "time"
)

func main() {
    timeString := time.Now().Format("2006-01-02 15:04:05")
    fmt.Println(timeString)
    fmt.Println(time.Now().Format("2019-08-20 18:05:32"))
}

Here is some ref for golang’s time format: https://yourbasic.org/golang/format-parse-string-time-date-example/ Any idea what’s the result before actually running it?

What? You know the trick? Oh, man, don’t waste your time here, skip this one and go have a beer 🍺

The magic date

As the reference showing above, I was thinking “uhmm, so we can just imagine and give an example about the date what it should look like, then golang will help us to format it properly? that’s awesome!”

But when come acroos in the real scenario…

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// result -> 
2019-08-23 23:06:54
23089-08-230 88:54:1123

What’s the… 23089-08-230… ???

Investigation

After bit research, it turns out that’s actually a “fixed” value that should be exactly showing there as “2006-01-02”, it cannot be anything else.

ref: https://gobyexample.com/time-formatting-parsing

ref: https://programming.guide/go/format-parse-string-time-date-example.html

But why???

More research…

Well, nothing is more clearly than the source code itself…

ref: https://github.com/golang/go/blob/master/src/time/format.go

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const (
	_                        = iota
	stdLongMonth             = iota + stdNeedDate  // "January"
	stdMonth                                       // "Jan"
	stdNumMonth                                    // "1"
	stdZeroMonth                                   // "01"
	stdLongWeekDay                                 // "Monday"
	stdWeekDay                                     // "Mon"
	stdDay                                         // "2"
	stdUnderDay                                    // "_2"
	stdZeroDay                                     // "02"
	stdUnderYearDay                                // "__2"
	stdZeroYearDay                                 // "002"
	stdHour                  = iota + stdNeedClock // "15"
	stdHour12                                      // "3"
	stdZeroHour12                                  // "03"
	stdMinute                                      // "4"
	stdZeroMinute                                  // "04"
	stdSecond                                      // "5"
	stdZeroSecond                                  // "05"
	stdLongYear              = iota + stdNeedDate  // "2006"
	stdYear                                        // "06"
	stdPM                    = iota + stdNeedClock // "PM"
	stdpm                                          // "pm"
	stdTZ                    = iota                // "MST"
	stdISO8601TZ                                   // "Z0700"  // prints Z for UTC
	stdISO8601SecondsTZ                            // "Z070000"
	stdISO8601ShortTZ                              // "Z07"
	stdISO8601ColonTZ                              // "Z07:00" // prints Z for UTC
	stdISO8601ColonSecondsTZ                       // "Z07:00:00"
	stdNumTZ                                       // "-0700"  // always numeric
	stdNumSecondsTz                                // "-070000"
	stdNumShortTZ                                  // "-07"    // always numeric
	stdNumColonTZ                                  // "-07:00" // always numeric
	stdNumColonSecondsTZ                           // "-07:00:00"
	stdFracSecond0                                 // ".0", ".00", ... , trailing zeros included
	stdFracSecond9                                 // ".9", ".99", ..., trailing zeros omitted

	stdNeedDate  = 1 << 8             // need month, day, year
	stdNeedClock = 2 << 8             // need hour, minute, second
	stdArgShift  = 16                 // extra argument in high bits, above low stdArgShift
	stdMask      = 1<<stdArgShift - 1 // mask out argument
)

So it covers everything for the time format…

How it calculates the time, it shows here:

ref: https://github.com/golang/go/blob/master/src/time/format.go in func “nextStdChunk”

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func nextStdChunk(layout string) (prefix string, std int, suffix string) {
	for i := 0; i < len(layout); i++ {
		switch c := int(layout[i]); c {
		case 'J': // January, Jan
			if len(layout) >= i+3 && layout[i:i+3] == "Jan" {
				if len(layout) >= i+7 && layout[i:i+7] == "January" {
					return layout[0:i], stdLongMonth, layout[i+7:]
				}
				if !startsWithLowerCase(layout[i+3:]) {
					return layout[0:i], stdMonth, layout[i+3:]
				}
			}

		case 'M': // Monday, Mon, MST
			if len(layout) >= i+3 {
				if layout[i:i+3] == "Mon" {
					if len(layout) >= i+6 && layout[i:i+6] == "Monday" {
						return layout[0:i], stdLongWeekDay, layout[i+6:]
					}
					if !startsWithLowerCase(layout[i+3:]) {
						return layout[0:i], stdWeekDay, layout[i+3:]
					}
				}
				if layout[i:i+3] == "MST" {
					return layout[0:i], stdTZ, layout[i+3:]
				}
			}

		case '0': // 01, 02, 03, 04, 05, 06, 002
			if len(layout) >= i+2 && '1' <= layout[i+1] && layout[i+1] <= '6' {
				return layout[0:i], std0x[layout[i+1]-'1'], layout[i+2:]
			}
			if len(layout) >= i+3 && layout[i+1] == '0' && layout[i+2] == '2' {
				return layout[0:i], stdZeroYearDay, layout[i+3:]
			}

		case '1': // 15, 1
			if len(layout) >= i+2 && layout[i+1] == '5' {
				return layout[0:i], stdHour, layout[i+2:]
			}
			return layout[0:i], stdNumMonth, layout[i+1:]

		case '2': // 2006, 2
			if len(layout) >= i+4 && layout[i:i+4] == "2006" {
				return layout[0:i], stdLongYear, layout[i+4:]
			}
			return layout[0:i], stdDay, layout[i+1:]

		case '_': // _2, _2006, __2
			if len(layout) >= i+2 && layout[i+1] == '2' {
				//_2006 is really a literal _, followed by stdLongYear
				if len(layout) >= i+5 && layout[i+1:i+5] == "2006" {
					return layout[0 : i+1], stdLongYear, layout[i+5:]
				}
				return layout[0:i], stdUnderDay, layout[i+2:]
			}
			if len(layout) >= i+3 && layout[i+1] == '_' && layout[i+2] == '2' {
				return layout[0:i], stdUnderYearDay, layout[i+3:]
			}

		case '3':
			return layout[0:i], stdHour12, layout[i+1:]

		case '4':
			return layout[0:i], stdMinute, layout[i+1:]

		case '5':
			return layout[0:i], stdSecond, layout[i+1:]

		case 'P': // PM
			if len(layout) >= i+2 && layout[i+1] == 'M' {
				return layout[0:i], stdPM, layout[i+2:]
			}

		case 'p': // pm
			if len(layout) >= i+2 && layout[i+1] == 'm' {
				return layout[0:i], stdpm, layout[i+2:]
			}

		case '-': // -070000, -07:00:00, -0700, -07:00, -07
			if len(layout) >= i+7 && layout[i:i+7] == "-070000" {
				return layout[0:i], stdNumSecondsTz, layout[i+7:]
			}
			if len(layout) >= i+9 && layout[i:i+9] == "-07:00:00" {
				return layout[0:i], stdNumColonSecondsTZ, layout[i+9:]
			}
			if len(layout) >= i+5 && layout[i:i+5] == "-0700" {
				return layout[0:i], stdNumTZ, layout[i+5:]
			}
			if len(layout) >= i+6 && layout[i:i+6] == "-07:00" {
				return layout[0:i], stdNumColonTZ, layout[i+6:]
			}
			if len(layout) >= i+3 && layout[i:i+3] == "-07" {
				return layout[0:i], stdNumShortTZ, layout[i+3:]
			}

		case 'Z': // Z070000, Z07:00:00, Z0700, Z07:00,
			if len(layout) >= i+7 && layout[i:i+7] == "Z070000" {
				return layout[0:i], stdISO8601SecondsTZ, layout[i+7:]
			}
			if len(layout) >= i+9 && layout[i:i+9] == "Z07:00:00" {
				return layout[0:i], stdISO8601ColonSecondsTZ, layout[i+9:]
			}
			if len(layout) >= i+5 && layout[i:i+5] == "Z0700" {
				return layout[0:i], stdISO8601TZ, layout[i+5:]
			}
			if len(layout) >= i+6 && layout[i:i+6] == "Z07:00" {
				return layout[0:i], stdISO8601ColonTZ, layout[i+6:]
			}
			if len(layout) >= i+3 && layout[i:i+3] == "Z07" {
				return layout[0:i], stdISO8601ShortTZ, layout[i+3:]
			}

		case '.': // .000 or .999 - repeated digits for fractional seconds.
			if i+1 < len(layout) && (layout[i+1] == '0' || layout[i+1] == '9') {
				ch := layout[i+1]
				j := i + 1
				for j < len(layout) && layout[j] == ch {
					j++
				}
				// String of digits must end here - only fractional second is all digits.
				if !isDigit(layout, j) {
					std := stdFracSecond0
					if layout[i+1] == '9' {
						std = stdFracSecond9
					}
					std |= (j - (i + 1)) << stdArgShift
					return layout[0:i], std, layout[j:]
				}
			}
		}
	}
	return layout, 0, ""
}

It has that date hard-coded inside of the function to recogonize the way you want the time format to be by using the case condition. Which… makes sense…

But why 2006-01-02 15:04:05? lol~~ Still no idea, maybe they just like this, all we need to do is remember this and use it to format the time in golang… let’s wish they are not changing it to anything else… 🙏 🙏 🙏