How to deal with time zones between Android and PHP/MySQL? - php

Question

I am building backend queue system. My app's users need to automatically fetch data from server around 08:00:00 AM, individually for each time zone.
Every user needs to be assigned to a specific time a day. He can fetch data only at this time as the app uses API that has specific calls-per-minute limits.
How do I synchronize clients with server?
NOTE
I ran into specific problems, and solved it already. I am posting the solution right away as a complete answer that combines many answers I found on SO while solving it.

Answer 1

Core of the solution
For clarity use time values in UTC that is supported in each Java/PHP/MySQL, because:
Although GMT and UTC share the same current time in practice, there is a basic difference between the two:
GMT is a time zone officially used in some European and African countries. The time can be displayed using both the 24-hour format (0 - 24) or the 12-hour format (1 - 12 am/pm).
UTC is not a time zone, but a time standard that is the basis for civil time and time zones worldwide. This means that no country or territory officially uses UTC as a local time.
source
It gives you simple solution as once you use UTC, you only need to convert it to server's or clients' time zone for display purposes.
Managing client's time zone
You need to send client's time zone to backend to calculate what time do you want him to call API. You want to convert 08:00:00 local time to UTC, but here's a trick, because there are incompatible time zones' strings between Java and PHP.
// Java/Android
SimpleDateFormat sdf = new SimpleDateFormat("z");
I live in Poland, and using the code above I get 2 different values depending on seasons (CET for winter time and CEST for summer time).
// PHP
$tz1 = new DateTimeZone('CET');
$tz2 = new DateTimeZone('CEST');
The problem is that when I pass it to PHP, CET works perfectly as it's supported time zone string, but CEST is not.
To unify your code, you need to use:
// Java/Android
SimpleDateFormat sdf = new SimpleDateFormat("ZZZZ");
which gives you a time zone likethis:
GMT+01:00 // for CET
GMT+02:00 // for CEST
Remember that when you send it in URL like http://api.domain.com?timezone=GTM+02:00, you need to change + into %2B as timezone converted to GTM 02:00 won't work in PHP.
Calculating queue time for users
Once you get client's time zone, in PHP you convert 08:00:00 AM local time to UTC.
$tz = new DateTimeZone('GMT+02:00');
$dt = new DateTime('2017-03-30 08:00:00', $tz);
$dt->setTimezone(new DateTimeZone('UTC'));
echo $dt->format('H:i:s');
// echoes 06:00:00
Then you store calculated value in MySQL at type TIME column. You don't need to care about time zone in the database as TIME and DATE types are time zone independent.
Setting alarm at calculated UTC time
You get 06:00:00 as a response in the app, and you set AlarmManager using Calendar object like this:
// set UTC as a time zone
Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
cal.setTime(new Date());
long timeNow = cal.getTimeInMillis();
// set 06:00:00
cal.set(Calendar.MILLISECOND, 0);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.HOUR_OF_DAY, 6);
// make sure to set alarm in future
long timeAlarm = cal.getTimeInMillis();
if (timeAlarm <= timeNow) {
cal.add(Calendar.HOUR_OF_DAY, 24);
}
alarm.setRepeating(AlarmManager.RTC_WAKEUP,
cal.getTimeInMillis(),
24*60*60*1000, pintent);