Convert a unix (milli) timestamp to year, month, day ... without using the standard library

unix-to-time.go

package main

import (
	"fmt"
	"time"
)

func main() {
	fmt.Println(unixToTime(time.Now().UnixMilli()))
}

// unixToTime takes a unix millisecond timestamp (time.UnixMilli())
// parses it and returns year, month, day, hour, minute, second (UTC).
//
// WARNING: ONLY USE something dodgy like this if there's an extremely
// specific reason not to use the standard library.
func unixToTime(timestamp int64) (int, int, int, int, int, int) {
	var (
		daysPerMonth = []int{0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}

		days = int(timestamp / 86400000)
		year = 1970
	)

	daysPerYear := 365
	for {
		dy := getDaysPerYear(year)
		if days < dy {
			break
		}

		days -= dy
		year++
		daysPerYear = dy
	}

	month := 1
	for {
		dm := daysPerMonth[month]
		// Adjust Feb day count for leapyear.
		if month == 2 && daysPerYear == 366 {
			dm = 29
		}

		if days < dm {
			break
		}

		days -= dm
		month++
	}

	var (
		day              = int(days) + 1
		secondsRemaining = int(timestamp / 1000 % 86400)
		hour             = secondsRemaining / 3600
		minute           = (secondsRemaining / 60) % 60
		second           = secondsRemaining % 60
	)

	return year, month, day, hour, minute, second
}

func getDaysPerYear(year int) int {
	if year%400 == 0 || (year%4 == 0 && year%100 != 0) {
		return 366
	}
	return 365
}