My ES6 variant produces a string like this 2020-04-05_16:39:45.85725. Feel free to modify the return statement to get the format that you need:
const getDateStringServ = timestamp => {
const plus0 = num => `0${num.toString()}`.slice(-2)
const d = new Date(timestamp)
const year = d.getFullYear()
const monthTmp = d.getMonth() + 1
const month = plus0(monthTmp)
const date = plus0(d.getDate())
const hour = plus0(d.getHours())
const minute = plus0(d.getMinutes())
const second = plus0(d.getSeconds())
const rest = timestamp.toString().slice(-5)
return `${year}-${month}-${date}_${hour}:${minute}:${second}.${rest}`
}