get begin time and end time in Golang.

day.go

package main

import (
	"fmt"
	"time"
)

func main() {
	now := time.Now()
	fmt.Println(
		now.Truncate(time.Hour*24), 
		now.Truncate(time.Hour*24).Add(time.Hour*23+time.Minute*59+time.Second*59), 
		now,
	)
}

month.go

package main

import (
	"fmt"
	"time"
)

func main() {
	now := time.Now()
	fmt.Println(
		now.AddDate(0, 0, 1-now.Day()).Truncate(time.Hour*24),
		now.AddDate(0, 1, -now.Day()).Truncate(time.Hour*24).Add(time.Hour*23+time.Minute*59+time.Second*59),
		now,
	)
}

// You can edit this code!
// Click here and start typing.
package main

import (
	"fmt"
	"time"
)

func MonthDays(firstDayOfMonth time.Time) int {
	diff := firstDayOfMonth.AddDate(0, 1, 0).Sub(firstDayOfMonth)
	return int(diff.Hours()) / 24
}

func main() {
	t, err := time.ParseInLocation("2006-01", "2019-12", time.Local)
	if err != nil {
		panic(err)
	}
	fmt.Println("Hello, 世界", MonthDays(t))
}

thanks for sharing