json - How to stringify dates with timezones properly in javascript?

Question

In javascript, I have a date in UTC, and I want to stringify it and parse it but maintain it's UTC. I did this code

var f = { f : new Date("Mon May 27 2019 20:11:13 GMT-0400 (Eastern Daylight Time)")}

undefined

JSON.stringify(f)

"{"f":"2019-05-28T00:11:13.000Z"}"

JSON.parse(JSON.stringify(f))

{f: "2019-05-28T00:11:13.000Z"}

You can see that after I stringified it, it changed to the next day. And then when I parse it, it kept it as a string and even of the next day. I want it so that after I parse it, I get back the Date object of Mon May 27 2019 20:11:13 GMT-0400 (Eastern Daylight Time).

Does anyone know what's wrong?

Thanks

Answer 1

You can see that after I stringified it, it changed to the next day

No, it didn't. "Mon May 27 2019 20:11:13 GMT-0400 (Eastern Daylight Time)" was parsed to a time value of 1559002273000, which defines a moment in time that is that many milliseconds after 1970-01-01T00:00:00Z. It's equivalent to 2019-05-28T00:11:13.000Z (and other timestamps in other time zones). E.g. on a system set to an eastern Australian timezone you'd get "Tue May 28 2019 10:11:13 GMT+1000 (AEST)".

I want it so that after I parse it, I get back the Date object of Mon May 27 2019 20:11:13 GMT-0400 (Eastern Daylight Time)

The date object has no knowledge of the timezone of the original timestamp, it just stores the time value. The string produced by toString just uses the host system timezone setting to generate a "local" timestamp in the format specified by ECMA-262. Note that the timezone name is implementation dependent and since they aren't standardised, the names and abbreviations differ between implementations. E.g. Safari shows "AEST" and Firefox "Australian Eastern Standard Time".

You can use toLocaleString to generate timestamps for different timezones, but the date object doesn't know what the original timezone was and the format may not be what you want.

Also, toLocaleString uses IANA timezone identifiers (e.g. Africa/Kinshasa), which relate to geographic locations that are used to deduce the applicable timezone rather than common names like "Eastern Daylight Time" which are not standardised and can be ambiguous or obscure. The IANA designators mean things like daylight saving and historic timezone offset changes can be applied more easily than with other designators.