Question

I write a function to reverse UTC time to local time

function utcToLocal(utc){
    var t = new Date(Number(utc));
    d = [t.getFullYear(), t.getMonth(), t.getDate()].join('/');
    d += ' ' + t.toLocaleTimeString();
    return d;
}

but i can’t confirm this code is right?

Answer 1

You should be able to convert the UTC timestamp into a local date and just subtract the local offset (which is in minutes), so:

function utcToLocal(utc){
    // Create a local date from the UTC string
    var t = new Date(Number(utc));

    // Get the offset in ms
    var offset = t.getTimezoneOffset()*60000;

    // Subtract from the UTC time to get local
    t.setTime(t.getTime() - offset);

    // do whatever
    var d = [t.getFullYear(), t.getMonth(), t.getDate()].join('/');
    d += ' ' + t.toLocaleTimeString();
    return d;
}

Where I am, the offset is -600, so I need to subtract -36,000,000 ms from UTC time (which actually adds 36,000,000 ms).

Edit

I may have misunderstood the question.

The internal value of a javascript date instance is a UTC time clip in milliseconds. So if utc is such a time (e.g. 2012-08-19T00:00:00Z is 1345334400000), then the OP will create a date instance based on that value and toLocaleTimeString will show an implementation dependent string of the local time for the supplied UTC time.

So if the local timezone offset is say -6hrs, then alert(new Date(1345334400000))) show something like Sat Aug 18 2012 18:00:00 GMT-600.

I was assuming that the OP wanted to set the local time to the same time as the UTC time, e.g. that 2012-08-19T00:00:00Z would become 2012-08-19T00:00:00 local.