Your example is complicated by 2 factors:
1: Using separate columns for date and time, rather than a single column
2: Using British format for the date
I have never tried to calculate elapsed time in Excel, but spent the last 2 hours learning. The following is the best I can do in the allotted time.
The pic below shows your chosen dates and times, but I have converted them to US date format: month/day/year rather than the British day/month/year. Additionally, I have put the time and date in the same cell, not in separate columns.
The formula in cell B4 is =a4-a3.
The formatting in column A is set to US English. The type is set to display 3/14/01 13:30.
The formatting in cell B4 where the formula resides is set to custom [h]:mm:ss. You can play around with that too.
The answer is 552 hours, 2 minutes, 31 seconds.
Note that the seconds are not displayed in cell a3, but are shown in the formula box at the top. The formula box also shows the time in traditional AM/PM format.
To use my method, you will have to convert your data to a single column, with date and time in the same cell, just as my sample shows. And you will have to use a US date format.
That's the best I can do. You can fiddle with the formatting and formulas and possibly come up with an improvement. Maybe someone else can add to this.