package main

import (
    "encoding/binary"
    "flag"
    "fmt"
    "log"
    "net"
    "time"
)

const ntpEpochOffset = 2208988800

type packet struct {
    Settings       uint8
    Stratum        uint8
    Poll           int8
    Precision      int8
    RootDelay      uint32
    RootDispersion uint32
    ReferenceID    uint32
    RefTimeSec     uint32
    RefTimeFrac    uint32
    OrigTimeSec    uint32
    OrigTimeFrac   uint32
    RxTimeSec      uint32
    RxTimeFrac     uint32
    TxTimeSec      uint32
    TxTimeFrac     uint32
}

func main() {
    var host string
    flag.StringVar(&host, "e", "182.92.12.11:123", "NTP host")
    flag.Parse()

    conn, err := net.Dial("udp", host)
    if err != nil {
        log.Fatalf("failed to connect: %v", err)
    }
    defer conn.Close()
    if err := conn.SetDeadline(time.Now().Add(15 * time.Second)); err != nil {
        log.Fatalf("failed to set deadline: %v", err)
    }

    req := &packet{Settings: 0x1B}

    if err := binary.Write(conn, binary.BigEndian, req); err != nil {
        log.Fatalf("failed to send request: %v", err)
    }

    rsp := &packet{}
    if err := binary.Read(conn, binary.BigEndian, rsp); err != nil {
        log.Fatalf("failed to read server response: %v", err)
    }

    secs := float64(rsp.TxTimeSec) - ntpEpochOffset
    nanos := (int64(rsp.TxTimeFrac) * 1e9) >> 32

    showtime := time.Unix(int64(secs), nanos)
    fmt.Printf("%v\n", showtime)
}

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