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.