Go time.Format and the Reference Time Approach

Go's Reference Time Approach

Go takes a fundamentally different approach to date formatting compared to every other mainstream language. Instead of abstract tokens like YYYY or %Y, Go formats a specific reference time in the layout you want.

The Magic Reference Time

Mon Jan 2 15:04:05 MST 2006

Or equivalently:

01/02 03:04:05PM '06 -0700

Each component is a specific number chosen to be unique:

• 1 (or 01) — Month (January = 1)
• 2 (or 02) — Day
• 3 (or 03, 15) — Hour (3 PM = 15:00)
• 4 (or 04) — Minute
• 5 (or 05) — Second
• 6 (or 06, 2006) — Year
• 7 (in -0700) — Timezone offset (MST = -0700)

Why This Approach?

The Go team designed this system so that the format string itself is a valid, readable date. When you see "2006-01-02", you immediately know the output will look like "2026-02-28" because the format IS a date. No lookup table required.

Common Layouts

package main

import (
    "fmt"
    "time"
)

func main() {
    t := time.Now()

    fmt.Println(t.Format("2006-01-02"))                    // 2026-02-28
    fmt.Println(t.Format("2006-01-02 15:04:05"))           // 2026-02-28 14:30:00
    fmt.Println(t.Format("Mon, 02 Jan 2006 15:04:05 -0700")) // RFC 2822
    fmt.Println(t.Format("January 2, 2006"))               // February 28, 2026
    fmt.Println(t.Format("3:04 PM"))                       // 2:30 PM
    fmt.Println(t.Format(time.RFC3339))                    // 2026-02-28T14:30:00+09:00
}

Predefined Layouts

Go's time package includes predefined layout constants:

Constant	Layout
time.Layout	01/02 03:04:05PM '06 -0700
time.ANSIC	Mon Jan _2 15:04:05 2006
time.RFC822	02 Jan 06 15:04 MST
time.RFC1123	Mon, 02 Jan 2006 15:04:05 MST
time.RFC3339	2006-01-02T15:04:05Z07:00

Common Gotchas

1. Padding: Use 02 for zero-padded, 2 for no padding, _2 for space-padded
2. 12h vs 24h: Use 3 or 03 for 12-hour, 15 for 24-hour
3. Timezone: -0700 for numeric offset, MST for abbreviation, Z0700 for Z-style UTC