To GMT or to not GMT

Toni and I are currently working on a project that has had as many as 9 developers working on it at one time, working from as many as four different locations, plus some developers also working from home. As a result, we've been doing a lot of work on VFE in recent months to make it more friendlier for team environment. One of the things we added to DBCX recently was the ability to write all of the metadata for a DBC out to an XML file and to load metadata from this XML file. This lets developers check in and check out individual DBCs and makes it easier for other developers to get their data dictionary changes without having to check in or check out the meta data for the entire project all at once. The project we're on currently has 47 DBCs, so you can imagine there's a lot of meta data as well.

Anyway, in working on this, I discovered that I needed to make sure that I properly dealt with different time zones. One of the offices is on West Coast time and well, Toni and I are likely to login from any number of time zones - and seeing as its February in Toledo right now, Hawaii Time sounds about perfect. So, in order to avoid any confusion related to timezones, I built the mechanisms for checking date and time of the changed metadata around GMT - Greenwich Mean Time. In a nutshell, in GMT it's always the same time everywhere in the world, no matter what time zone you're in - even if you're in Indiana or New Foundland. For more info on GMT, check this out.

I haven't done real well with posting code in the blog, so I put a file of the GMT related functions here.

There are 4 functions in this procedure file:

• GMTDateTime() - which returns the current GMTDateTime. There's a Windows API call to get this.
• GMTTimeDifference() - which returns the difference in seconds between the machine's local time and GMT. This function is used internally for the remaining functions.
• GMTToLocal(ltDateTime) - which converts GMT a to its local equivalent.
• LocalToGMT(ltDateTime) - which converts a local date & time to its GMT equivalent.

The code is commented pretty well, so I won't bore you with the details here.

This really got me thinking, should my applications be storing dates and times in GMT and then converting it to the current users local date and time for display purposes? It'd really depend on the application, but this is definitely something that I plan on giving somethought with my next application. Most of the time when we look at a date and time, we think of it in terms of our current locale, this could be a big issue in apps where the data comes from multiple locations and of course in web apps, where the users could be logging on at any time from any where.

So, I don't really have any guidelines or recommendations at this point for when GMT should or should not be used. If you have any, let me know. It's definitely something worth more thought.