Summing Datenum resets after 24 hours

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Hello all,
I am trying to sum datenum values, and return them to hours:mins:secs format when all complete. When the amount of time is relatively small, it works as expected, but once it rolls past 24 hours, it resets.
a = datenum('21:10:00','hh:MM:ss')
b = datenum('1:10:00','hh:MM:ss')
c = datestr(x+y,'hh:MM:ss')
c = '22:20:00'
x = datenum('21:10:00','hh:MM:ss')
y = datenum('7:10:00','hh:MM:ss')
z = datestr(x+y,'hh:MM:ss')
z = '04:20:00'
Anyone know how to get around that to output :
z = '28:10:00'
and also 3 digit hours if need-be.
Any help is greatly appreciated!

Accepted Answer

Stephen23 on 15 Dec 2020
Edited: Stephen23 on 15 Dec 2020
As Steven Lord wrote, serial date numbers are not the right tool for the task: amongst other things you will struggle against defining an appropriate pivot year (which is essential for the task that you are attempting) and as you have already found DATESTR will (by design) return the hours of a day, not an arbitrarily increasing hour count.
But you can easily do it yourself without serial date numbers, here using time in seconds:
v = [60*60,60,1];
a = v * sscanf('21:10:00','%d:%d:%d'); % seconds
b = v * sscanf( '1:10:00','%d:%d:%d'); % seconds
ans = '22:20:00'
x = v * sscanf('21:10:00','%d:%d:%d'); % seconds
y = v * sscanf( '7:10:00','%d:%d:%d'); % seconds
ans = '28:20:00'
function str = mytime(tmp)
spl = nan(1,3);
spl(3) = mod(tmp,60); % seconds
tmp = fix(tmp/60);
spl(2) = mod(tmp,60); % minutes
tmp = fix(tmp/60);
spl(1) = tmp ; % hours
str = sprintf('%02d:%02d:%02d',spl);
Steven Lord
Steven Lord on 15 Dec 2020
Instead of using sscanf you could directly call duration as of release R2018a. I know this doesn't help the original poster, but I'm including it as an FYI.
d = duration('21:10:00', 'InputFormat', 'hh:mm:ss')
d = duration

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More Answers (2)

Matt Gaidica
Matt Gaidica on 15 Dec 2020
I don't know if the datetime functions are going to allow that triple digit hour. Here's one approach.
z = sprintf('%03i:%02i:%02i',hour(x) + hour(y), minute(x+y), second(x+y))
Ruger28 on 15 Dec 2020
Update: the tiem commands are also not available in 2014a. Thanks again, though.

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Steven Lord
Steven Lord on 15 Dec 2020
Serial date numbers are not the right tool for this job. You want to use a duration array.
dur1 = duration(12, 23, 45)
dur1 = duration
dur2 = hours(67)+minutes(89)
dur2 = duration
68.483 hr
D = dur1 + dur2
D = duration
If you want you can control the display Format.
D.Format = 'h'
D = duration
80.879 hr
  1 Comment
Ruger28 on 15 Dec 2020
Thank you for your reply, but I am using 2014a, and duration was introduced in 2014b, unfortunately.

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