Go time.Now().UnixNano() convert to milliseconds?

Question

How can I get Unix time in Go in milliseconds?

I have the following function:

func makeTimestamp() int64 {
    return time.Now().UnixNano() % 1e6 / 1e3
}

I need less precision and only want milliseconds.

Answer 1

The 2021 answer:

As of go v1.17, the time package added UnixMicro() and UnixMilli(), so the correct answer would be: time.Now().UnixMilli()

For go v.1.16 and earlier:

Just divide it:

func makeTimestamp() int64 {
    return time.Now().UnixNano() / 1e6
}

1e6, i.e. 1 000 000, is the number of nanoseconds in a millisecond.

Here is an example that you can compile and run to see the output

package main

import (
    "time"
    "fmt"
)

func main() {
    a := makeTimestamp()

    fmt.Printf("%d \n", a)
}

func makeTimestamp() int64 {
    return time.Now().UnixNano() / 1e6
}

Answer 2

Keep it simple.

func NowAsUnixMilli() int64 {
    return time.Now().UnixNano() / 1e6
}

Answer 3

As @Jono points out in @OneOfOne's answer, the correct answer should take into account the duration of a nanosecond. Eg:

func makeTimestamp() int64 {
    return time.Now().UnixNano() / (int64(time.Millisecond)/int64(time.Nanosecond))
}

OneOfOne's answer works because time.Nanosecond happens to be 1, and dividing by 1 has no effect. I don't know enough about go to know how likely this is to change in the future, but for the strictly correct answer I would use this function, not OneOfOne's answer. I doubt there is any performance disadvantage as the compiler should be able to optimize this perfectly well.

See https://en.wikipedia.org/wiki/Dimensional_analysis

Another way of looking at this is that both time.Now().UnixNano() and time.Millisecond use the same units (Nanoseconds). As long as that is true, OneOfOne's answer should work perfectly well.

Answer 4

How can I get Unix time in Go in milliseconds?

Go 1.17 and above

No more divisions from nanoseconds. Starting from Go 1.17 you can just use Time.UnixMilli method directly:

// a deterministic date value
t := time.Date(2021, 7, 16, 0, 0, 0, 0, time.UTC)

m := t.UnixMilli()
fmt.Println(m) // 1626393600000

Playground: https://play.golang.org/p/JSExv5jw2ZW

Answer 5

At https://github.com/golang/go/issues/44196 randall77 suggested

time.Now().Sub(time.Unix(0,0)).Milliseconds()

which exploits the fact that Go's time.Duration already have Milliseconds method.

Answer 6

I think it's better to round the time to milliseconds before the division.

func makeTimestamp() int64 {
    return time.Now().Round(time.Millisecond).UnixNano() / (int64(time.Millisecond)/int64(time.Nanosecond))
}

Here is an example program:

package main

import (
        "fmt"
        "time"
)

func main() {
        fmt.Println(unixMilli(time.Unix(0, 123400000)))
        fmt.Println(unixMilli(time.Unix(0, 123500000)))
        m := makeTimestampMilli()
        fmt.Println(m)
        fmt.Println(time.Unix(m/1e3, (m%1e3)*int64(time.Millisecond)/int64(time.Nanosecond)))
}

func unixMilli(t time.Time) int64 {
        return t.Round(time.Millisecond).UnixNano() / (int64(time.Millisecond) / int64(time.Nanosecond))
}

func makeTimestampMilli() int64 {
        return unixMilli(time.Now())
}

The above program printed the result below on my machine:

123
124
1472313624305
2016-08-28 01:00:24.305 +0900 JST

Answer 7

Simple-read but precise solution would be:

func nowAsUnixMilliseconds(){
    return time.Now().Round(time.Millisecond).UnixNano() / 1e6
}

This function:

1. Correctly rounds the value to the nearest millisecond (compare with integer division: it just discards decimal part of the resulting value);
2. Does not dive into Go-specifics of time.Duration coercion — since it uses a numerical constant that represents absolute millisecond/nanosecond divider.

P.S. I've run benchmarks with constant and composite dividers, they showed almost no difference, so feel free to use more readable or more language-strict solution.