Say I have time for a given Olson/tz database timezone, for example, July 1st, 1973 at 15h 23min in Africa/Maputo.
How can I convert it to universal time UT and vice-versa in node.js? I have seen some libraries online, but the documentation is unclear. A complete code example is welcome.
Using moment.js with moment-timezone, and assuming ISO8601 formatting (you didn't specify).
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npm install moment-timezone
var moment = require('moment-timezone');
Converting from local time to UTC
var m = moment.tz("1973-07-01T15:23", "Africa/Maputo");
var s = m.toISOString(); // "1973-07-01T13:23:00.000Z"
or
var m = moment.tz("1973-07-01T15:23", "Africa/Maputo");
var s = m.utc().format(); // "1973-07-01T13:23:00.000+00:00"
The first form is more compact and is easiest if you are just sending the result over an API or saving it in a database. The second form is an example of using the utc function, which is more useful if you intend to pass a parameter to format to produce a string formatted differently, or if you intend to call some other function than format.
Converting from UTC to local time
var m = moment.utc("1973-07-01T13:23").tz("Africa/Maputo");
var s = m.format(); // "1973-07-01T15:23:00+02:00"
or
var m = moment("1973-07-01T13:23:00.000Z").tz("Africa/Maputo");
var s = m.format(); // "1973-07-01T15:23:00+02:00"
In the first form, I show that you can use the moment.utc function to parse a string that does not contain offset information and have it interpreted as UTC. This is the more explicit form.
The second form shows that when there is offset information (either Z or +00:00 forms) it is taken into account. Though the intermediate result is a moment object in local mode, the underlying timestamp is still UTC-based, and thus the conversion to the specified time zone still gives the same output.
