How to specify timestamp format when converting to human readable string in JS

I need to display a formatted date from a timestamp provided by Google Analytics Standard solution like

var date = new Date(timestamp * 1000);
var formatted = date.toString();

produces wrong value Jan 01 1970. That's because of timestamp format.

In PHP I can specify the timestamp format:

\DateTime::createFromFormat('Ymd', $timestamp);

How to do this in JS?

Solution

Since the dates you are receiving are formatted as YYYYMMDD, not as a Unix timestamp, you can parse it by extracting the year, month and date using String.prototype.slice.

var timestamp = '20170306',
  year = parseInt(timestamp.slice(0, 4), 10),
  month = parseInt(timestamp.slice(5, 6), 10),
  day = parseInt(timestamp.slice(7, 8), 10);
  // - 1 because the Date constructor expects a 0-based month
  date = new Date(Date.UTC(year, month - 1, day)),
  gmt = date.toGMTString(),
  local = date.toString();

console.log('GMT:', gmt); // Mon, 06 Mar 2017 00:00:00 GMT
console.log('Local:', local);

This assumes that the dates you are using are in UTC (which they likely are). Date.UTC creates a timestamp (in milliseconds since Unix epoch) and then feeds it into new Date() which uses it to create a Date object representing that time. .toGMTString() outputs the date formatted for the GMT timezone. To output it formatted in local time, use .toString() instead.