How can I subtract the times without considering the date in question?

I have a vector of time which are linked to dates but I only want to subtract the times from each other without considering the dates too. For example, I want to do 4:30 - 3:30 is equal to 1 hour and then 3:30 - 2:50 is equal to 40 minutes (0.6666667) and etc. I dont want Matlab to consider the date period basically.

I have this code for the time from 25-Apr to 01-May but it does take into account the date too:

out1m=24-(seconds(diff(datetime([T_Smooth{2,4};T_Smooth{3,4}])))/3600);

Can anyone help please? Thank you.

[Scott MacKenzie]

Just to clarify... What result do you expect if you subtract 3:30 on April 26 from 4:30 on April 27? 1 hour or 25 hours?

[Newbie]

1 hour

Answer 1

[Cris LaPierre]

Use the timeofday function.

% create vector of datetimes
date = datetime(2019,[4;4;5;5],[24;25;1;2],[4;3;2;4],[30;30;50;30],0)

date = 4×1 datetime array
   24-Apr-2019 04:30:00
   25-Apr-2019 03:30:00
   01-May-2019 02:50:00
   02-May-2019 04:30:00

% Take difference of the times.
dt = diff(timeofday(date))

dt = 3×1 duration array
   -01:00:00
   -00:40:00
    01:40:00

Use the hours function to turn hh:mm:ss into decimal hours if you want.

hours(dt)

ans = 3×1

-1.0000 -0.6667 1.6667

[Newbie]

Thanks, this works!

Answer 2

[Scott MacKenzie]

Here's one way to do this:

% test data (one month in 30-min increments)
dt = datetime(2021,4,1):minutes(30):datetime(2021,5,1);

% indices of datetime elements to compute hour difference between
n1 = 154; 
n2 = 255;

x = datenum(dt(n2) - dt(n1));
hrs = rem(x,1) * 24

hrs = 2.5000

[Newbie]

Thanks ever so much!

[Stephen23]

I would recommend avoiding mixing DATETIME/DURATION with the old deprecated date functions.

Using TIMEOFDAY is a much better approach.

[Scott MacKenzie]

Noted. Thanks.

BTW, @Stephen Cobeldick what deprecated date functions are you referring to? I only used datetime and minutes in my solution.

[Stephen23]

"what deprecated date functions are you referring to? I only used datetime and minutes in my solution."

You used DATENUM here:

https://www.merriam-webster.com/dictionary/deprecated: "3: to withdraw official support for or discourage the use of (something, such as a software product) in favor of a newer or better alternative."

The DATENUM documentation currently contains this note:

[Scott MacKenzie]

Oops, yes, I used datenum as well. Thanks for pointing that out. The language to discourage use of this function could be a tad stronger, however. Below is what appears for xlsread, as an example.