calculate the difference between two datetime values

I wish to calculate the difference between two datetime values. I converted them to "datenum" and then subtracted them and tried to use the "datetime" to get the results in the date and time format. however, the results yielded are incorrect and the dates it mentions is in December 1999. I have attached my code and the file. Any help would be appreciated. Thanks in advance.

[Jan]

The results are correct, but you expect something else: If the result of the subtraction is e.g. 1 s, a conversion to a datetime object means the first second of the year, which is treated as year 0 internally.

[Piyush Shaunak]

is there any way I can rectify that?

Answer 1

[Walter Roberson]

Just subtract the two datetime variables. The result will be a duration object. You can express it in particular time units by setting the Format property of the duration object, or you can use calls like hours() or seconds() to get the numeric equivalent relative to those units.

[Piyush Shaunak]

I did subtract the two datetime variables but on changing the answer to datetime form it yields in incorrect values (Dec 1999).

[Walter Roberson]

It does not make sense to convert the difference in datetimes back to a a datetime object.

If the start time is (say) June 19, 1973 18:21 and the end time is (say) June 19, 1973 18:31, then what output would you be hoping for?

[Peter Perkins]

To illustrate what Walter is suggesting:

>> A = readtable('Oxygenvelocity.csv');
Warning: Variable names were modified to make them valid MATLAB identifiers. The original names are saved in the
VariableDescriptions property. 
>> A
A =
  216×4 table
      DateAndTime       Subsurface_Dopto_     DateAndTime_1      Surface_Exo_
    ________________    _________________    ________________    ____________
    23/07/2015 14:30         14.046          23/07/2015 17:15         5.9    
    24/07/2015 04:00          9.278          24/07/2015 00:00        5.61    
    25/07/2015 00:00         12.375          25/07/2015 22:45        5.12    
    26/07/2015 01:00         13.031          26/07/2015 21:45        5.43    
    27/07/2015 00:30         14.615          27/07/2015 14:00        5.72    
>> A.TimeDiff = A.DateAndTime - A.DateAndTime_1
A =
  216×5 table
      DateAndTime       Subsurface_Dopto_     DateAndTime_1      Surface_Exo_    TimeDiff 
    ________________    _________________    ________________    ____________    _________
    23/07/2015 14:30         14.046          23/07/2015 17:15         5.9         -2:45:00
    24/07/2015 04:00          9.278          24/07/2015 00:00        5.61         04:00:00
    25/07/2015 00:00         12.375          25/07/2015 22:45        5.12        -22:45:00
    26/07/2015 01:00         13.031          26/07/2015 21:45        5.43        -20:45:00
    27/07/2015 00:30         14.615          27/07/2015 14:00        5.72        -13:30:00

A.TimeDiff is a duration that measures how many hours/minutes/seconds elapsed between the two datetimes. That seems like what you would want.

All your timestamps seem to be on the same day. If you were working with longer time spans, it's possible that you would want to use the between function, which can return elapsed time in terms of calendar units like months and days, rather than subtraction.