Time zone conversion

Unfortunately I don't know of any that will do that. It sounds easy enough but to my knowledge the only two time zones you can convert between using standard functions or easily available functions is local time and UTC.

Thus, you can convert from local time to UTC (GMT) and then you could change to a different timezone and change from UTC to that local timezone.

There are two problems here:

1. It is troublesome that you have to switch timezone and so on, it would be much better if the timezone had been a separate data structure.

2. How do you get the timezone for a different timezone? The truth is that your OS probably have tables and databases for all timezones and other info depending on place. However, those tables are used when you install the timezone for your system and thenafter they are more or less dead and not accessible. There is no API to my knowledge to access them. The installation program that does the selection knows their internal format and reads them but does so directly without using any API that would be available to user programs.

What would be nice to have is a function like:

tzptr = get_timezone("Berlin");

and the function would then return some timezone_t data type and you could then use a

struct tm * tmptr = localtime2(& time, tzptr);

function. Unfortunately there is no standard function for that sort of things.

Not even windows new framework .NET has functionality for this although it is a progress compared to old systems, they have a timezone data type but that class has only a static function to get the local timezone. Also, an obvious candidate would be a UTC timezone object and a static function that could do something like:

static timezone_t * TimeZone::GetTimeZone(const char * name);

where name could be strings like "PST" or "Los Angeles" and so on.

It is very unfortunate that it is this way, not even glibc which has done a big improvement in locale_t type functions which is a similar type of problem hasn't provided a solution for timezone problematic.

So, given that you get no support from the system, here is what you can do:

Make your own local time in your program. Just hard code all the info and data about various timezones that you know of. The real problem here is info such as when a specific timezone start daylight savings and when they stop it. Some places has standardized these things and you can algorithmically calculate when they start and stop daylight saving. Problem is that this isn't for all places and it is certainly not for all times. Historically various times where chosen and very often there was no algorithmic or other reason for when they decided to have daylight saving, it was simply a political decision made by some authority and you have to plot it into the table. What is worse: Historically daylight savings has even been different within the same nation from one region to another. US for example have traditionally had that different states changed to daylight savings at different dates. Causing all sorts of troubles in communication between them.

I believe that in recent times they do it all at the same time and the change to/from daylight saving is determined nationally but historically that wasn't how it was done.

So welcome to timezone hell :-)

If you want to make it simple though you can easily enough just plot down various time zone differences. When time is 6pm in new york it is midnight in central europe most of the time - the only exceptions are during daylight savings and in european countries that doesn't follow CET.

Using such a table you can thus convert the times properly at least for now. To make the change to daylight savings correct in the conversion you might discover that the code to handle that will suddenly blow your program up by several 1000 lines of code just to handle all the oddities around daylight savings etc.

Alf