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how can we determine the hours elapsed between two times. For example:

3:30PM in San Francisco and 7:30PM in Dubai

The part that I am unclear is that, is there a universal way of calculating the subtracted timespan, by taking time zones and countries in to account.

I am using C# as a primary language. Any help will be deeply appreciated.

Thanks in advance.

user153410
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3 Answers3

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You asked about:

"3:30PM in San Francisco and 7:30PM in Dubai"

If all you know is the time and location, then you have a problem because not all time zones are fixed entities. Dubai is fixed at UTC+4, but San Francisco alternates between UTC-8 for Pacific Standard Time and UTC-7 for Pacific Daylight Time. So a date is also required to make this conversion.

The following will give you the difference using the current date in the first time zone:

TimeZoneInfo tz1 = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");
TimeZoneInfo tz2 = TimeZoneInfo.FindSystemTimeZoneById("Arabian Standard Time");

DateTime today = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, tz1).Date;

DateTime dt1 = new DateTime(today.Year, today.Month, today.Day, 15, 30, 0); // 3:30 PM
DateTime dt2 = new DateTime(today.Year, today.Month, today.Day, 19, 30, 0); // 7:30 PM

TimeSpan elapsed = TimeZoneInfo.ConvertTimeToUtc(dt1, tz1) -
                   TimeZoneInfo.ConvertTimeToUtc(dt2, tz2);

(Note that the ID "Pacific Standard Time" is the Windows time zone identifier for the US Pacific Time zone, and despite the name, covers both PST and PDT.)

But even this is not necessarily the best approach because "today" isn't the same in all time zones. At (almost) any point in time, there are two different days in operation worldwide. This illustration from Wikipedia demonstrates this well.

                                                        Date Animation

It's possible that the "current" date for the source time zone is not the same date currently in the end time zone. Without additional input, there's not much you can do about that. You need to know the date in question for each value.

Matt Johnson-Pint
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If you use the type DateTimeOffset you can avoid some of the pitfalls of using local time because it can represent the local time in any time zone:

var dateTimeOffsetSanFrancisco = DateTimeOffset.Parse("9/8/2014 3:30PM -0700");
var dateTimeOffsetDubai = DateTimeOffset.Parse("9/8/2014 7:30PM +0400");
var elapsedHours = (dateTimeOffsetDubai - dateTimeOffsetSanFrancisco).TotalHours;
Martin Liversage
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    Please note that using .Hours will not always return the correct amount: It will 'Roll Over' at times greater than one day (Ie. 25 hours will return 1). TotalHours will give you the actual 'total hours elapsed'. – t0yk4t Sep 08 '14 at 07:50
  • @MelanieSchmidt: Oops, that was a silly typo. Fixed now. Thanks. – Martin Liversage Sep 08 '14 at 08:05
  • Yes, this is also a good approach - When you know the dates and offsets in advance. – Matt Johnson-Pint Sep 08 '14 at 17:25
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The simplest way is to convert both DateTime objects to UTC and then subtracting them:

var utcSanFran = sfLocal.ToUniversalTime();
var utcDubai = dubaiLocal.ToUniversalTime();
var elapsed = dubaiLocal - sfLocal;

Assuming that Dubai > San Francisco.

To get the total hours elapsed use:

elapsed.TotalHours;

Please note that this will apparently only work for the local computer's time zone. See http://msdn.microsoft.com/en-us/library/system.timezoneinfo(v=vs.110).aspx for a better way to do this.

t0yk4t
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  • Ummm... no. This isn't a good solution. When you call `ToUniversalTime`, it can only convert from the local time zone of the machine it's running on. Since a single computer can't be in San Francisco and Dubai at the same time, this code won't work regardless of where it is run. – Matt Johnson-Pint Sep 08 '14 at 17:03
  • Well the least you could've done is point to the correct solution. http://msdn.microsoft.com/en-us/library/system.timezoneinfo(v=vs.110).aspx should provide all the info OP needs. – t0yk4t Sep 08 '14 at 17:08
  • I can only type so fast. :) – Matt Johnson-Pint Sep 08 '14 at 17:23