*: fix min timestamp

[csuzhangxc]

previously:

package main

import (
    "fmt"
    "time"
)

func main() {
    ts := time.Date(1970, 1, 0, 0, 0, 0, 0, time.UTC).Unix()
    fmt.Println("ts", ts)

    t := time.Unix(ts, 0).UTC()
    fmt.Println("t", t)
}

ts -86400
t 1969-12-31 00:00:00 +0000 UT

[csuzhangxc]

@mahjonp @you06 PTAL