a file's mtime is not reliable when rotating log by datetime

[wangcn]

test code

package main

import (
	"os"
	"time"
	"fmt"
)


func main() {
	t1 := time.Now()
	fmt.Println(t1)
	file, _ := os.Create("test.txt")
	stat, _ := file.Stat()
	t2 := stat.ModTime()
	fmt.Println(t2)
	fmt.Println(t2.Sub(t1))
}

t2 < t1.
my env: go1.7.3+openSUSE42.1+xfs
If rotating a log by comparing mtime and "now", the historical log may be overwritten multiple times and you lose it.