How to get a date in format "yyyy-mm-dd" from the day of year number and year in Golang?

I am trying to format a date in the format "yyyy-mm-dd" using as input the number of the day in year (0 to 365).

As input : 035 and 2021
Output : 2021-02-04

I really don't know if the Go's time library can help doing this simply.

Anyone have any idea of to do it ?

If you mean day number, then that usually goes from 1 to 365 (or 366 for leap year). Because only then day 35 of 2021 corresponds to 2021-02-04.

Just use time.Date, it will convert an out-of-range day parameter correctly:

> The month, day, hour, min, sec, and nsec values may be outside their usual ranges and will be normalized during the conversion. For example, October 32 converts to November 1.

Example:

package main

import (
	"fmt"
	"time"
)

func main() {
	year, day := 2021, 35
	dt := time.Date(year, 1, day, 0, 0, 0, 0, time.Local)
	fmt.Println(dt.Format("2006-01-02"))
}

Live demo

Prints:

2021-02-04

If you actually mean day number 0 to 364 (or 365 for leap year), then change day to day+1 like so:

	dt := time.Date(year, 1, day+1, 0, 0, 0, 0, time.Local)

This does it:

package main

import (
   "fmt"
   "time"
)

func dayOfYear(year, days int) time.Time {
   return time.Date(year, 1, 1+days, 0, 0, 0, 0, time.UTC)
}

func main() {
   t := dayOfYear(2021, 35)
   fmt.Println(t)
}

Although, your math is off, as 35 days would be February 5, as the year starts
January 1, not January 0.

https://golang.org/pkg/time#Date

This might be what you are looking for, make sure to replace the zone with the zone in which you need the time string to be displayed.

package main

import (
	"fmt"
	"time"
)

func main() {
	var layout = "2006-01-02"
	zone := time.UTC
	year := 2021
	day  := 35
	
	time := time.Date(year,1,day,0,0,0,0,zone)
	converted := time.Format(layout)
	fmt.Println(converted)
}