Times to GMT

Question

Suppose I have times, in the past (many before 1970), in many different time zones. No matter what the time, zone, or dst situation, I need to convert these to the correct UTC. Not for the purpose of printing, but for internal, programmatic calculations.
So, for example, let's say we have June 5, 1966, at 10:24:51 EDT. I need to get this into UTC so that I can perform calculations that require time in UTC.
I've seen a lot of code in the archives around similar questions, but most of them assume that you're trying to format this correctly to print, or that the formulae for daylight savings time is the same as it is currently. One of the things I want to be able to set manually is whether DST was in effect or not. For example, in Germany during the 1940s, they had "war time" which was actually a two hour dst. it seems that the TimeZone class automatically determines dst, which could be a problem for these old dates.
Ten Duke dollars to anyone who can show me the full code for handling this problem.

Answer 1

Hi,
In my work as Chief Scientist working with researchers in diverse fields, I have had to address this problem myself. There are a couple of approaches to consider, one of the ones I'll share is more elegant but has its own drawbacks...
In any method you choose, the key here is to understand that any 'date-time' field is incomplete information if it does not also include the GMT offset (or UDT, as is slightly more correct these days). You may know the GMT offset value because you know the source of the data, or because it was specified to you. ...I have found that there are a number of places around the world that have non-whole-hour offsets, so you can't just add or subtract hours and have your code be correct the world over. I therefore use a minutes offset value.
Note also that the direction of the offset "doesnt' matter" so long as YOU know when to add and when to subtract your offset value. And it helps if you choose the + or - sign methodology in connection with user expectations (which, I have learned, are not universal).
The more graceful method, which I don't use, by the way, is to create a "cronon type" object that includes the GMT (UDT) offset, and for which there are various methods to get local time and put local time and set and fetch the offset value. This gives you most of what you want - maybe you'll add comparative methods in your class, or maybe you'll leave that to the application code that uses your new cronon type - that's up to you.
Another way, which is what I use, is to assign an offset value to sources of data and then contextually switch to and from local time or "Universal Time" depending, again, on the context of the call. This is a fine solution when you are mixing and matching datasets that came from different sources but which have a well defined context of useage. WARNING: YOU MUST HAVE A WELL DEFINED SEMANTIC FOR CONTEXTUAL USE FOR THIS STRATEGY TO WORK RELIABLY! That is, go with the cronon class route if you don't have or can't rely upon useage paterns.
Hope this helps,
Richard