Timezone issues with MySQL vs PHP - php

Question

Background: I have some PHP code that calculates the next time a recurring message should be sent according to various rules. I also have a sanity page that double checks that the next recurrence is correct. Due to a timezone problem that I am not sure how to resolve, the sanity page is showing some problems that I am not sure if they are actually problems.
I believe that these are showing up because the (future) date is during BST (British Summertime) but the Europe/London timezone is currently on GMT.
I created a very simplified test page to debug and test one example. The code is as follows:
// Not included: DB connection setup, which includes a query to set the MySQL session timezone to the current timezone for Europe/London as calculated by PHP
// Copied from a comment at http://php.net/manual/en/function.date-default-timezone-get.php
function timezone_offset_string( $offset )
{
return sprintf( "%s%02d:%02d", ( $offset >= 0 ) ? '+' : '-', abs( $offset / 3600 ), abs( $offset % 3600 ) );
}
$offset = timezone_offset_get( new DateTimeZone( date_default_timezone_get() ), new DateTime() );
// query_multiple() is a function to get a single result from the database and return it as the specified PDO type
$db = query_multiple("SELECT NextRecurrence, UNIX_TIMESTAMP(NextRecurrence) AS NextTS, IF(##session.time_zone = 'SYSTEM', ##system_time_zone, ##session.time_zone) AS DBTZ FROM RecurringMessages WHERE RecurID=96 LIMIT 1", "", "", PDO::FETCH_ASSOC);
print "DB Date of NextRecurrence : ".$db['NextRecurrence']."<br>";
print "DB timestamp of NextRecurrence : ".$db['NextTS']."<br>";
print "DB timezone : ".$db['DBTZ']."<br>";
print "PHP timezone and offset: " .date_default_timezone_get().", ".timezone_offset_string( $offset ) . "<br>";
print "PHP date of NextTS : ".date('Y-m-d H:i:s', $db['NextTS'])."<br>";
The output of this page is as follows:
DB Date of NextRecurrence : 2017-05-24 10:00:00
DB timestamp of NextRecurrence : 1495620000
DB timezone : +00:00
PHP timezone and offset: Europe/London, +00:00
PHP date of NextTS : 2017-05-24 11:00:00
As you can see, the PHP date is one hour different, even though the PHP and MySQL timezones are the same. As mentioned above, I believe that this is because the date given is during BST and Europe/London is currently GMT.
If I put 1495620000 into http://www.epochconverter.com/, it tells me that it is Wed, 24 May 2017 10:00:00 GMT
There are two things that I am currently unsure of
Is the current database entry correct? When it comes to May 24th 2017, will this message be sent at 10:00 or 11:00? The PHP code, which is used for sending the message, sets the session time zone for MySQL right after connecting to match the current timezone for Europe/London.
If the database entry is correct, how do I get the PHP date function to give me the correct output for any future date? If the PHP date function is correct, how do I (generically) update the MySQL record with the correct date? All I know is that it should be sent on May 24th 2017 at 10:00. Currently the query updates with UPDATE SET NextRecurrence='2017-05-24 10:00:00' ...

Answer 1

I figured it out (I think).
Because MySQL is currently at GMT, I need to present the Y-m-d H:i:s formatted date and time to MySQL in GMT, even if the date is during BST.
// $ts contains the correct unix timestamp
$offsetnow=date('Z');
$tsoffset=date('Z', $ts);
$diff=$offsetnow-$tsoffset;
// Add the difference to the timestamp so that the PHP date returns the appropriately presented Y-m-d H:i:s to MySQL, which as of writing this is GMT.
$ts+=$diff;
$query="UPDATE Recurrences SET NextRecurrence='".date('Y-m-d H:i:s', $ts)."' ...
If anyone sees a mistake in what I am doing, please let me know!