How to subtract 'datetime' from 'datetime' in MATLAB GUI ?

Hi, I am designing a program to calculate the time different between two cities.

The operation is: subtracting second city time from the first one. I used the bellow code to get the cities time and convert it into 'string':

city1 = datestr(datetime('now','TimeZone','local',...
                         'Format','dd-MM-yyyy HH:mm:ss Z'));
city1.TimeZone = 'America/New_York';
city2 = datestr(datetime('now','TimeZone','local',...
                         'Format','dd-MM-yyyy HH:mm:ss Z'));
city2.TimeZone = 'Africa/Khartoum';

The output will be in a formatted string as:

        city1 =
                '25-Aug-2018 19:34:29'

        city2 =
                '25-Aug-2018 13:34:29'

The above is working properly but I can not subtract them in this form.

My question is: How to subtract the string forms above and get 4 cells output as "06:00" ?

[dpb]

The above cannot "work properly"...

>> c1=datestr(datetime('now','TimeZone','local','Format','dd-MM-yyyy HH:mm:ss Z'));
>> c1.TimeZone = 'America/New_York';
Field assignment to a non-structure array object. 
>>

In fact, ML tries to address the variable as a struct but can't do that with a char string.

city1, city2 (I shortened to c1,c2) are char() arrays; you can't assign a property value to them.

OTOMH I'm not sure how to use the datetime object to display the time zone difference between two locations but I'm sure it's doable without manipulating the date strings...

Answer 1

[dpb]

The problem is converting to strings instead of just comparing...

What will work to compute the offset including DST if in effect is--

v=now;                  % return the local current time as datenum(*)
t1=datetime(v,'ConvertFrom','datenum','TimeZone','America/New_York','Format','d-MMM-y HH:mm:ss Z');
t2=datetime(v,'ConvertFrom','datenum','TimeZone','Africa/Khartoum','Format','d-MMM-y HH:mm:ss Z');
u=t2-t1
u = 
duration
 -7:00:00

The above sets the local time a variable before calling the conversion factors to ensure there isn't a rollover between the two calls to datetime that would introduce a small error in the result if were to occur.

(*) Interesting shortcoming that TMW hasn't introduced a datetime-aware routine to parallel the old standby that returns a datenum.

ADDENDUM

An alternative would be to use the timezones table data--

A=timezones('Africa');            % get the Africa table
N=timezones('America');           % and Americas, too
Z1=N(contains(N.Name,'New_York'); % retrieve TZ of specific interest
Z2=A(contains(A.Name,'Khartoum');
u=Z2.UTCOffset+Z2.DSTOffset-(Z1.UTCOffset+Z1.DSTOffset) % offset including DTS
u =
   7

The problem with this is that I don't believe the table will update the DTS offset with time of year; it's just the value used in the location. So, this is probably not viable actual solution lacking a test for whether DST is in effect in both locations at the time of interest.