Calculate hours elapsed from 2 times

Hello,

I have a cell array with two columns. It looks like the following:

thetime = ['9:30PM' , '12:30pm', '11:30am' ; '8:00am', '6-6:30am', '7:20am']

I want to find out how many hours have passed from items in column 1 to those in column 2 (same row).

Before subtracting starting time from ending time, I have tried to convert am/pm times to 24hour-time using:

hoursspent = datestr([thetime{:}],'HH:MM');

but this returns: Cannot convert input into specified date string. DATENUM failed.

I have also tried:

hoursspent = datenum([thetime{:}],'HH:MM');

but this returns a 1x1 cell with the number 7.3487e+05

What am I doing wrong? Any ideas as to what I can try instead?

Thank you so much for your time and help!

Answer 1

[Sumit Tandon]

Try using the DATEVEC command to find difference between two dates/times.

For example:

>> a = datevec('9:30 PM');
>> b = datevec('9:40 PM');
>> b-a
ans =
   0     0     0     0    10     0

DATENUM converts date information to serial date format.

[Milou]

When I tried that I got the following:

Error using datevec (line 213) Failed to lookup month of year.

is this because I only have times in my data?

[Sumit Tandon]

What did you specify as input? DATEVEC should assume default year, month and day if only time is specified.

[Milou]

i specified the columns (1 and 3) with time data in them.

hours1 = data(:,1);

hours3 = data(:,3);

hours1 = datevec(hours1);

hours3 = datevec(hours3);

AmountTime = hours1 - hours3;

[Sumit Tandon]

Are you sure about the data going into DATEVEC command? Use the debugger to check whats in hours1 and hours3. DATEVEC does work with cell arrays if you are using that.

Note that variable "thetime" in your original question is a normal char array and not a cell array. Use {} for cell arrays.

[Milou]

also, i just realized that your example above shows:

9:40pm - 9:30pm = 10.

I want a function that calculates the time elapsed between those times, ie. 12 hours.

Is there any way to do this?

Answer 2

[Star Strider]

See the ‘etime’ function.

Example:

          thetime = {'9:30PM' , '12:30pm', '11:30am' ; '8:00am', '6-6:30am', '7:20am'}'
          time2 = datevec(thetime(1,:), 16)
          Td_s = etime(time2(1,:),time2(2,:))     % Elapsed time in seconds
          Td_h = Td_s/3600                        % Elapsed time in hours

The ‘16’ is a predefined time format. See ‘Numeric Identifiers for Predefined Formats’ about half way down the ‘Date and Time Functions’ page: http://www.mathworks.com/help/techdoc/matlab_prog/bspgcx2-1.html

[Milou]

when i try that, the output is a 1x1 cell array, so only one number.

does this mean that only two items (one in each) is evaluated?

do you know how i can resolve this?

thank you for your time.

[Star Strider]

*NOTE:* I did my best to format the Command Window output here as ‘code’ but while it displayed correctly in ‘Preview’ it doesn't seem to have carried over to the posted version after I clicked ‘Save’.

__________________________________________________________________________

My pleasure.

When you say ‘output’, what is the output you refer to?

When I ran the code I quoted, I got these results:

thetime =

'9:30PM' '8:00am'

'12:30pm' '6-6:30am'

'11:30am' '7:20am'

time2 =

Columns 1 through 5

2.0120e+003 1.0000e+000 1.0000e+000 21.0000e+000 30.0000e+000

2.0120e+003 1.0000e+000 1.0000e+000 8.0000e+000 0.0000e+000

Column 6

0.0000e+000

0.0000e+000

Td_s =

48.6000e+003

Td_h =

13.5000e+000

What part of my code fragment didn't work for you?

Please note that I intend it as an example. You will have to adapt it to your code, particularly because I have no idea what you want to do with '6-6:30am'. The MATLAB functions will not understand what you mean with that so you will have to change it to something ‘datevec’ will understand before you run your code.

[Milou]

i formatted to fit my code: it works but only for the first item (first row of adjacent columns 1 and 2), i want a function that gives me the time difference for all adjacent elements of the array (2 columns of 10 elements each, so i am trying to find a function that goes through all of them and returns 10 differences).

[Star Strider]

It is probably best in your situation to use a ‘for’ loop. (I am currently running a very large ‘GlobalSearch’ problem in the background in MATLAB that will probably run for several more hours, so I am not able to test the code.)

I suggest you consider something like:

--------------------------------------------------------

for k1 = 1:size(thetime,1)

      time2 = datevec(thetime(k1,:), 16);

      Td_s = etime(time2(1,:),time2(2,:));

      Td_h(k1) = Td_s/3600;

end

--------------------------------------------------------

I put extra blank lines in my loop and lines around my code in case it again refuses to format as code.

You still need to deal with the '6-6:30am' problem. As it is now, the ‘datevec’ function will throw an error when it gets to that row.

The ‘datevec’ function assumes the current year, month, and day, and that the times reflect consecutive days. To be accurate, you need to provide the year, month, and day for each time as well.

[Milou]

even with the loop and changing '6-6:30am' to '6:30am', i get the following error:

Error using datevec (line 213) Failed to lookup month of year.

where do i provide the year, month, and day for each time?

[Star Strider]

How far into the loop do you get your error? Does it read most of your times?

When I ran my code (on MATLAB 2012a), it assumed the current year, month, and day, then used your time data, as in the Command Window output I quoted earlier (but couldn't format correctly). According to the documentation I quoted to you in my original Anwwer, ‘datevec’ does not require any more than the time. That is why I used the pre-defined format ‘16’.

Since I cannot reproduce your error in my code (in part because my GlobalSearch problem is still running 13½ hours after I started it and I cannot test anything until it finishes), I do not know what to advise you to do.

[Star Strider]

After stopping GlobalSearch (after 18 hours without results) I ran the code I gave you with ‘6-6:30pm’ changed to ‘6:30pm’. After changing this line to guarantee a column vector:

Td_h(k1,:) = Td_s/3600;

got this output:

Td_h =

      13.5000e+000
       6.0000e+000
       4.1667e+000

Since I cannot reproduce your error, I cannot help you figure out what is wrong with your code and why my code won't run in your application.