Retrieve Natural Time Intervals in MySQL for the client Time Zone - mysql

Question

I have a PHP-MySQL API with Time Zone Support. My database is in UTC and all date time calculations are made in UTC time, just converting to local time when data is displayed to the user.
The problem is when trying to retrive data in a natural time interval for a user in a different time zone, for example weeks, months, years and so on. The DB column is type DateTime and store dates in UTC. If I need all rows grouped by month and just use the UTC date stored in the database I will get some wrong rows, if the Time Zone differences made some rows shift its month.
Example: row with value 2015-05-01 00:00:00 is in May in UTC, but should be in April for any user in a negative Time Zone.
So using UTC is not the solution here. I need first to convert those dates to client Time Zone.
Which aproach can I use to overcome this problem?
As an example this is view groups some data by week:
SELECT `individuals_id`,
Str_to_date(Concat(Yearweek(`sessions_date`, 5), ' Monday'), '%X%V %W') AS `sessions_date`,
`protocol_index`,
`zone_index`,
Avg(`asymmetry_dom`) AS `asymmetry_dom`,
Avg(`asymmetry_rl`) AS `asymmetry_rl`,
FROM `sessions_data_view`
GROUP BY `individuals_id`,
Yearweek(`sessions_date`, 5),
`protocol_index`,
`zone_index`
ORDER BY `individuals_id`,
Yearweek(`sessions_date`, 5),
`protocol_index`,
`zone_index`
The problem is that Yearweek() should have different output for a row depending on the user time zone. It is not posible to use the column sessions_date in UTC if want to give consistent result to the user.
Rigth now I do not know the user Time Zone, but this should not be a limitation, since the app is in its desing phase and anything can be changed.
The API is a PHP application getting HTTP requests. It talks to a PHP Database class that wraps all queries to the MariaDB database. All response from the aPI is given as JSON, dates formated as UTC strings. The data is shown via a web application. DateTime Javascript objects are responsible to convert the responses from the API to correct dates for the client time zone.

Answer 1

Firstly, the only reliable way you have of getting the client's timezone is through javascript, assuming their computer's time-settings are correct.
Solution One - Javascript to PHP
I suggest collecting the users timezone with the following javascript:
var timeZone = new Date().getTimezoneOffset();
You will need to store this value in the database. You then format the UTC time with PHP code similar to this:
$date = new DateTime();
$timezone = new DateTimeZone('Australia/Sydney');
$date->setTimezone($timezone);
$formatted_date = $date->format('Y-m-d H:i:s');
Solution Two - PHP to Javascript
You convert a UTC string into a unix timestamp with the following:
$time = strtotime($utc);
You then return that to the browser, and have javascript format it like this:
var date = new Date(timestamp * 1000),
datevalues = [
date.getFullYear(),
date.getMonth()+1,
date.getDate(),
date.getHours(),
date.getMinutes(),
date.getSeconds(),
];
Good luck!
edit: For converting MySQL for use with queries, use CONVERT_TZ, the format being:
CONVERT_TZ('date/time field','starting timezone','ending timezone')
For example:
select *, DATE_FORMAT(CONVERT_TZ(str_to_date(timestamp),'UTC','US/Eastern'), '%m/%d/%y %h:%i %p') as 'Date - Eastern' FROM table`