Question

I use a legacy DMS application, who stores dates using GMT 0 (Greenwich) as default time zone and applies 1 hour spread to it. I have to display with GridView these records and I need to apply a kind of transformation according to the relative location where my system runs (as example London, Bahamas).

Looking on how the legacy system works with dates, I designed the following algorithm to display the dates properly (my code is based on asp.net / C#):

//Example for Bahamas, GMT: -5 Hours as offset, I should add 4 hours to the DB date
//Example for London,  GMT:  0 Hour  as offset, I should add 1 hour  to the DB date
DateTime dateToDisplay;
int spreadHours  = 0;

TimeZone cur = TimeZone.CurrentTimeZone;
DaylightTime daylight = cur.GetDaylightChanges(dateFromDb.Year);
DateTime start = daylight.Start;
DateTime end = daylight.End;

if (dateFromDb.CompareTo(start) <= 0 || dateFromDb.CompareTo(end) >= 0)
{
   spreadHours  = -1 - (cur.GetUtcOffset(dateFromDb).Hours);                
}
else
{
   spreadHours  = - (cur.GetUtcOffset(dateFromDb).Hours);                
}

dateToDisplay = dateFromDb.AddHours(spreadHours);

However I am not sure if with this process I can cover all the cases or whether there could be a better solution to achieve the same result.

Can anyone confirm my idea or suggest a better path?

Answer 1

In general from .NET 3.5 you could/should use the TimeZoneInfo class,

in fact to convert from an UtcDateTime to a local time all you need to do is this:

// here static text but you can initialize the TimeZoneInfo with any Id, check MSDN for this:
// http://msdn.microsoft.com/en-us/library/system.timezoneinfo.aspx

string nzTimeZoneKey = "New Zealand Standard Time";

TimeZoneInfo nzTimeZone = TimeZoneInfo.FindSystemTimeZoneById(nzTimeZoneKey);
DateTime nzDateTime = TimeZoneInfo.ConvertTimeFromUtc(utcDateTime, nzTimeZone);

you can check this other question and answers here in SO:

Convert UTC/GMT time to local time