Issue #353: Date Ranges should honour timezone settings.

[lukecarbis]

The problem here is that the date picker uses the date of the visitor’s
computer. This means that we have to take the site GMT offset, and add
the local computer’s GMT offset, and then compare the current date with
the offset date.

Resolves #353

[frankiejarrett]

@lukecarbis Travis is failing due to lack of spacing in this line: get_option( 'gmt_offset' )

[westonruter]

#OMGBBQ 😉

[frankiejarrett]

@lukecarbis Unfortunately, this PR isn't quite working for me yet.

1. I spoof my local env time to be 8pm (which equals 1am GMT since I am -0500).
2. I double check in Settings > General that my local time and GMT are in fact different dates (3/27 vs. 3/28).
3. I clear my browser cache by testing in incognito mode to ensure the latest JS is being used.
4. I select "Today" from the predefined date list and it puts 2014/03/28 into the date fields instead of 2014/03/27.

[c3mdigital]

@fjarrett @lukecarbis I had this same javascript offset problem on some scheduling feature I worked on. I'm adding the code I had to use to get it working right in case it might help here.

getRegionTime: function () {
            var date = new Date(), UTC = date.getTime() + (date.getTimezoneOffset() * 60000);
            return new Date(UTC + (3600000 * Ajax_Schedules.getOffset()));
        },

This relied on the getOffset method which just converted the users timezone to + - hours from UTC then checking if it's daylight savings using this function and adding the extra hour if true.

        is_dst: function () {
            Date.prototype.stdTimezoneOffset = function () {
                var jan = new Date(this.getFullYear(), 0, 1);
                var jul = new Date(this.getFullYear(), 6, 1);
                return Math.max(jan.getTimezoneOffset(), jul.getTimezoneOffset());
            };
            Date.prototype.dst = function () {
                return this.getTimezoneOffset() < this.stdTimezoneOffset();
            };
            var today = new Date();
            return today.dst();
        },

Another function that used some of the built in datepicker methods:

        showLocalTime: function () {
            var time_div = $('.local-time'), date = Ajax_Schedules.getRegionTime(), sp = ' ',
                hour = 12 < date.getHours() ? date.getHours() - 12 : date.getHours(),
                _hour = (0 === hour) ? 12 : hour, format = 'D M d yy',
                minute = date.getMinutes() + '',
                _minute = (1 === minute.length) ? '0' + minute : minute,
                _time = $.datepicker.formatDate(format, date).toUpperCase() + ' at ' + _hour + ':' + _minute;

            time_div.html( '<time datetime="' + $.datepicker.formatDate('yy-mm-dd', date) + '" class="txt-regular right">' + _time + '</time>');
        },

[lukecarbis]

@c3mdigital Thanks!

@fjarrett Good pickup. I hadn't thought to test the dropdown menu, I was only looking at the datepicker fields.

[frankiejarrett]

@lukecarbis When I select Yesterday from the dropdown the start date is correct, but the end date is incorrect.

[lukecarbis]

Okay - I've found a better way of implementing timezone support in carbon. I had to get the wordpress timezone string (e.g. Australia/Brisbane) but if it didn't exist (e.g. GMT +10), then I had to search for the string based on the offset.