I have a server in Usa and I have clients in different parts of the world, Australia, South america, Usa, Canada, Europe.
So I need to send notification of events one hour before the event take place.
So In sql server I have a table with different events those events are stored in Utc(2015-12-27 20:00:00.0000000). and in other table the timezone that belongs to every event ("Australia/Sydney").
So how could I calculate in a query when to send the notifications? or maybe I would have to do it with a server side language.
Could any one could help me with a possible solution.
Thanks
You've asked very broadly, so I can only answer with generalities. If you need a more specific answer, please edit your question to be more specific.
A few things to keep in mind:
Time zone conversions are best done in the application layer. Most server-side application platforms have time zone conversion functions, either natively or via libraries, or both.
If you must convert at the database layer (such as when using SSRS or SSAS, or complex stored procs, etc.) and you are using SQL Server, then there are two approaches to consider:
SQL Server 2016 CTP 3.1 adds native support for time zone conversions via the AT TIME ZONE statement. However, they work with Windows time zone identifiers, such as "AUS Eastern Standard Time", rather than IANA/Olson identifiers, such as the "Australia/Sydney" you specified.
You might use third-party support for time zones, such as my SQL Server Time Zone Support project, which does indeed support IANA/Olson time zone identifiers. There are other similar projects out there as well.
Regardless of whether you convert at the DB layer or at the application layer, the time zone of your server should be considered irrelevant. Always get the current time in UTC rather than local time. Always convert between UTC and a specific time zone. Never rely on the server's local time zone setting to be anything in particular. On many servers, the time zone is intentionally set to UTC, but you should not depend on that.
Nothing in your question indicates how you plan on doing scheduling or notifications, but that is actually the harder part. Specifically, scheduling events into the future should not be based on UTC, but rather on the event's specific time zone. More about this here.
You might consider finding a library for your application layer that will handle most of this for you, such as Quartz (Java) or Quartz.Net (.NET). There are probably similar solutions for other platforms.
You should read the large quantity of material already available on this subject here on Stack Overflow, including the timezone tag wiki and Daylight saving time and time zone best practices.