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I have a situation where I am always returned the date from the server as a UK date time string.

E.g. '2020-07-19 16:40:00'

This would be 4:40PM in UK at +01:00, or 3:40PM UTC.

I want to be able to convert this time from GMT to the local time on the computer;

If I do this when in the UK...

var date = new Date('2020-06-19 16:40:00 GMT');

it returns Fri Jun 19 2020 17:40:00 GMT+0100 (British Summer Time)

Which is an hour out.

If I do the date in winter time (without daylight savings), this is correct.

var date = new Date('2020-01-19 16:40:00 GMT');

returns Sun Jan 19 2020 16:40:00 GMT+0000 (Greenwich Mean Time)

Is there a way I can correctly adjust this to always give the correct time regardless of what timezone the computer is set in, based on UK clock times.

Thanks in advance

user3284707
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1 Answers1

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As I understand your question, you don't know if the timestamp from the server is GMT or BST as the offset isn't included. You can work it out using plain JS but it's somewhat kludgy and error prone, see Calculate Timezone offset only for one particular timezone.

It would be much better to get the server to use an ISO 8601 format supported by ECMAScript and either send the offset or always use UTC/GMT.

If that isn't an option, you can use a library like Luxon to specify the location (and hence offset rules) to use for parsing, e.g.

let DateTime = luxon.DateTime;

['2020-07-19 16:40:00', // BST +1
 '2020-01-19 16:40:00'  // GMT +0
].forEach(ts => console.log(
  DateTime.fromFormat(ts, 'yyyy-LL-dd HH:mm:ss', {zone: 'Europe/London'}))
);
<script src="https://cdn.jsdelivr.net/npm/luxon@1.24.1/build/global/luxon.min.js"></script>

PS Don't forget to always tell the parser the format to parse.

RobG
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  • Thanks so much for your answer, you understand correctly. I will try this out later with luxon and get back to you. – user3284707 Jul 02 '20 at 10:20