Timezone to UTC: How do I add duration vector to datetime vector?

I want to use the new tzoffset (r2014b+) to make a function to change a datenum time vector to UTC timezone but I've encountered a strange behavior while doing operations on a datetime vector. I would like to know if I'm missing something here.

time=['2015/03/08 00:00';'2015/03/08 01:00';'2015/03/08 03:00';'2015/03/08 04:00'];
Time=datenum(time,'yyyy/mm/dd HH:MM')
date=datetime(Time,'ConvertFrom','datenum','Timezone','America/Montreal')
[UTC_Offset,dst]=tzoffset(date)
Date=date-UTC_Offset

Should give

   08-Mar-2015 05:00:00
   08-Mar-2015 06:00:00
   08-Mar-2015 07:00:00
   08-Mar-2015 08:00:00

But there's 1 extra hour being added before the daylight saving time change.

   08-Mar-2015 06:00:00
   08-Mar-2015 07:00:00
   08-Mar-2015 07:00:00
   08-Mar-2015 08:00:00

The same behavior is present when doing:

   date(1)+duration(5,0,0)

So I believe something is up with the datetime vector. Does anybody else get this or I just did something wrong?

Answer 1

[Peter Perkins]

Frank, what's up is Daylight Saving Time, which occurred on March 8th this year. I'm guessing you knew that, but got mixed up in the time arithmetic. Here's what's going on. You have this:

>> time = ['2015/03/08 00:00';'2015/03/08 01:00';'2015/03/08 03:00';'2015/03/08 04:00'];
>> Time = datenum(time,'yyyy/mm/dd HH:MM')
Time =
                    736031
          736031.041666667
                736031.125
          736031.166666667

datenums don't know anything about time zones or DST. Those times mean whatever you want them to mean. Since you then do this ...

>> date = datetime(Time,'ConvertFrom','datenum','Timezone','America/Montreal')
date = 
   08-Mar-2015 00:00:00
   08-Mar-2015 01:00:00
   08-Mar-2015 03:00:00
   08-Mar-2015 04:00:00

... I assume you mean that those datenums are to be interpreted as times in the America/Montreal time zone. But what you've left out is this:

>> date.Format = [date.Format ' z']
date = 
   08-Mar-2015 00:00:00 UTC-5
   08-Mar-2015 01:00:00 UTC-5
   08-Mar-2015 03:00:00 UTC-4
   08-Mar-2015 04:00:00 UTC-4

The second and third times are only an hour apart. Then this ...

>> [UTC_Offset,dst] = tzoffset(date);
>> Date = date - UTC_Offset
Date = 
   08-Mar-2015 06:00:00 UTC-4
   08-Mar-2015 07:00:00 UTC-4
   08-Mar-2015 07:00:00 UTC-4
   08-Mar-2015 08:00:00 UTC-4

... adds 5 hours to the first and second times, but adds only 4 hours to the third and fourth times. But you're doing the addition in the America/Montreal time zone. Adding 5 hours to the first two puts them after the DST shift @2am, which makes them look like you added 6 hours. In other words, your array is not UTC, it's shifted America/Montreal. The easiest thing to do to get UTC times is this:

>> date.TimeZone = 'UTC'
date = 
   08-Mar-2015 05:00:00 UTC
   08-Mar-2015 06:00:00 UTC
   08-Mar-2015 07:00:00 UTC
   08-Mar-2015 08:00:00 UTC

Hope this helps.

[Frank B.]

Thanks Peter! I knew I had something wrong here. Thanks for this line. Didn't saw that when I skim through the doc.

date.TimeZone = 'UTC'

Cheers