時刻の「時」以下を切り捨てる処理

now := time.Now()
ut := now.Unix()
_, offset := now.Zone()
day := time.Unix((ut/86400)*86400-int64(offset), 0)

day := time.Unix(((ut+int64(offset))/86400)*86400-int64(offset), 0).In(now.Location())

package zerotime

import (
	"fmt"
	"time"
)

//fmt パッケージを使って文字列整形する
func TrancateByFmt(t time.Time) time.Time {
	day, _ := time.Parse(
		"2006-01-02 15:04:05 -0700",
		fmt.Sprintf("%s 00:00:00 %s", t.Format("2006-01-02"), t.Format("-0700")),
	)
	return day
}

//time.Formatを使って文字列整形する
func TrancateByFormat(t time.Time) time.Time {
	day, _ := time.Parse(
		"2006-01-02 15:04:05 -0700 MST",
		t.Format("2006-01-02 00:00:00 -0700 MST"),
	)
	return day
}

//差分を計算してtime.Time.Add関数を使う
func TrancateByCalc(t time.Time) time.Time {
	nanosecond := time.Duration(t.Nanosecond())
	second := time.Duration(t.Second())
	minute := time.Duration(t.Minute())
	hour := time.Duration(t.Hour())
	dur := -1 * (nanosecond + second*time.Second + minute*time.Minute + hour*time.Hour)
	return t.Add(dur)
}

//time.Date関数で0時0分を再設定する
func TrancateByDate(t time.Time) time.Time {
	return time.Date(t.Year(), t.Month(), t.Day(), 0, 0, 0, 0, t.Location())
}

//Unix-Timeを使う
func TrancateByUnixTime(t time.Time) time.Time {
	_, offset := t.Zone()
	return time.Unix(((t.Unix()+int64(offset))/86400)*86400-int64(offset), 0).In(t.Location())
}

func BenchmarkTrancateByFmt(b *testing.B) {
	loc, _ := time.LoadLocation("America/New_York")
	t := time.Now().In(loc)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_ = zerotime.TrancateByFmt(t)
	}
}

func BenchmarkTrancateByFormat(b *testing.B) {
	loc, _ := time.LoadLocation("America/New_York")
	t := time.Now().In(loc)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_ = zerotime.TrancateByFormat(t)
	}
}

func BenchmarkTrancateByCalc(b *testing.B) {
	loc, _ := time.LoadLocation("America/New_York")
	t := time.Now().In(loc)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_ = zerotime.TrancateByCalc(t)
	}
}

func BenchmarkTrancateBydate(b *testing.B) {
	loc, _ := time.LoadLocation("America/New_York")
	t := time.Now().In(loc)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_ = zerotime.TrancateByDate(t)
	}
}

func BenchmarkTrancateByUnixTime(b *testing.B) {
	loc, _ := time.LoadLocation("America/New_York")
	t := time.Now().In(loc)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_ = zerotime.TrancateByUnixTime(t)
	}
}

$ go test -bench Trancate -benchmem
goos: linux
goarch: amd64
pkg: zerotime
BenchmarkTrancateByFmt-4        	  987878	      1096 ns/op	     224 B/op	       8 allocs/op
BenchmarkTrancateByFormat-4     	 1233117	       953 ns/op	     176 B/op	       4 allocs/op
BenchmarkTrancateByCalc-4       	33584716	        31.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkTrancateBydate-4       	11734460	        99.7 ns/op	       0 B/op	       0 allocs/op
BenchmarkTrancateByUnixTime-4   	77463438	        15.5 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	zerotime	7.601s