Javascript：您能像新的Date（localTime +'GMT'）这样将本地时间转换为GMT吗？

Question

I currently have a var dateNow as '10 1 2016' and var timeNow as '05:15:50' and was able to convert to UTC like so var dateTimeNow = Date(dateNow + timeNow + ' UTC');

Would now like to convert it to GMT, so is it as simple as just changing ' UTC' to ' GMT' ? So like, var dateTimeNow = Date(dateNow + timeNow + ' GMT'); ?

Answer 1

If you have a date like "10 1 2016" (Which I guess is 10 January, 2016) and a time like "05:15:50" then you can create a date for that time in a timezone with a zero offset as an ISO 8601 formatted string: "2016-01-10T15:15:50Z" or "2016-01-10T15:15:50+0000".

An ISO 8601 extended format string should be parsed correctly in modern implementations, however in general it's not a good idea to parse strings with the Date constructor (or Date.parse, they are equivalent for parsing) due to variances in implementations. If you have a single format, it can be parsed in a couple of lines. Alternatively, use one of the many Date libraries that have a parser and formatter and remember to always give the parser the format of the string you wish it to parse.

As for converting a local time to UTC, you must know the time zone offset of the local time, otherwise you have no datum to adjust it.

To "convert" a local date like 10 January, 2016 at 05:15:50 to UTC (where "local" is whatever the host system time zone is set to) is a simple as:

 var d = new Date(2016,0,10,5,15,50); console.log('Local: ' + d.toLocaleString() + '\\nUTC: ' + d.toISOString()); 

Note that toLocaleString is entirely implementation dependent, often ignores browser and system settings and produces a different result in different implementations.

This allows the host to consider the current system time zone settings and apply them when creating the date. ECMAScript Date objects have a time value that is based on UTC, so they are inherently UTC.