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,
	)
}

Only work in local time is UTC……

package main

import (
	"fmt"
	"time"
)

// TimeIn returns the time in UTC if the name is "" or "UTC".
// It returns the local time if the name is "Local".
// Otherwise, the name is taken to be a location name in
// the IANA Time Zone database, such as "Africa/Lagos".
func TimeIn(t time.Time, name string) (time.Time, error) {
	loc, err := time.LoadLocation(name)
	if err == nil {
		t = t.In(loc)
	}
	return t, err
}

func main() {
	for _, name := range []string{
		"",
		"Local",
		"Asia/Shanghai",
		"America/Metropolis",
	} {
		t, err := TimeIn(time.Now(), name)
		if err == nil {
			fmt.Println(t.Location())
			fmt.Println(
				t.AddDate(0, 0, 1-t.Day()).
					Truncate(time.Hour*24),
				t.AddDate(0, 0, 1-t.Day()).
					AddDate(0, 1, -1).
					Truncate(time.Hour*24).
					Add(time.Hour*23+time.Minute*59+time.Second*59,
				t,
			)
		} else {
			fmt.Println(name, "<time unknown>")
		}
	}

}

New method

package main

import (
	"fmt"
	"time"
)

// TimeIn returns the time in UTC if the name is "" or "UTC".
// It returns the local time if the name is "Local".
// Otherwise, the name is taken to be a location name in
// the IANA Time Zone database, such as "Africa/Lagos".
func TimeIn(t time.Time, name string) (time.Time, error) {
	loc, err := time.LoadLocation(name)
	if err == nil {
		t = t.In(loc)
	}
	return t, err
}

func InMonth(dt time.Time) (st, et time.Time) {
	st = time.Date(dt.Year(), dt.Month(), 1, 0, 0, 0, 0, dt.Location())
	et = time.Date(dt.Year(), dt.Month(), 1, 23, 59, 59, 0, dt.Location())
	et = et.AddDate(0, 1, -1)
	return
}

func main() {
	now := time.Now()
	fakelocal, _ := TimeIn(now, "Asia/Shanghai")

	monthBegin, monthEnd := InMonth(fakelocal)
	fmt.Println(monthBegin, monthEnd)
}

// 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