SharePoint Designer - How to get current server datetime through JavaScript REST?

The accepted answer here is wrong because it does not account for DST (daylight savings time).

Using _spPageContextInfo also doesn't always work because that's not necessarily populated (especially if you're making a provider-hosted app).

To get the current time in the timezone of a particular site, you can use the /_api/web/RegionalSettings/TimeZone/utcToLocalTime endpoint, passing it the current UTC time. You can also use this to convert any time to the server's timezone:

function parseDate(isoString) {
    var parts = /(\d+)-(\d+)-(\d+)T(\d+):(\d+):(\d+)/.exec(isoString);

    return new Date(parts[1], parts[2] - 1, parts[3], parts[4], parts[5], parts[6]);
}

function convertToSiteTime(webUrl, dateTime)
{
    return $.ajax({
        url: webUrl + 
             "/_api/web/RegionalSettings/TimeZone/utcToLocalTime(@date)?@date='"+ 
             dateTime.toUTCString() + 
             "'",
        method: "GET",
        headers: { "Accept": "application/json; odata=verbose" }
    }).then(function (data){
        return parseDate(data.d.UTCToLocalTime);
    });
}

Example usage:

convertToSiteTime('https://sharepoint.mydomain.com/my/site/', new Date())
    .then(function (result) { console.log(result); })

You can also expand upon the above to determine the time offset between the user's local time and the site's local time, and then use that to calculate dates thereafter. Note that this would not be a good idea for calculating dates that are far apart because the offset could be different at different parts of the year depending on DST:

function getTimeOffset() {
    var referenceTime = new Date();

    return convertToSiteTime('https://qdabra.sharepoint.com', referenceTime)
        .then(function(result) { 
            return (referenceTime - result) / 1000 / 60; 
        });
}

getTimeOffset().then(function (offset) { 
    console.log('The local time is', offset, 'minutes ahead of the site\'s time.'); 
});

Vadim's answer does not get the server time. It will return the time set on the local computer (OS Time). You can verify this very easily by changing the date on your computer and see the time that comes up.

The data from that endpoint only includes time zone information.

Looking for an easier solution. Hopefully we don't have to include sp.js to use _spPageContextInfo just to get the time :-/

I am calling from a blank - non templated page.

Once you have access to _spPageContextInfo, you can get the server time from:

_spPageContextInfo.serverTime

It will give you something like this:

2017-07-18T20:49:59.0827749Z

See how to get the _spPageContextInfo on a blank ASPX page from:

Bare minimum page to get _spPageContextInfo loaded

Cheers!

I tried the accepted answer but it seems to give wrong time. I found this post which explains a better way and it seems to work for me always.

You use _spPageContextInfo.clientServerTimeDelta to get the time delta and then add that to current date time.

var currentServerDateTime = new Date(new Date().getTime() + _spPageContextInfo.clientServerTimeDelta);